To clarify a bit on what Grimdour said:
OU = Overused
UU = Underused
NU = Neverused
NFE = Not fully evolved
A team belongs to a tier depending on what Pokemon are on it. For example, if it has 6 Pokemon from the UU (Underused) tier, it would probably be considered a UU team, although that's not necessarily the case. It would be able to play in UU tiers as well as every tier above UU - that is, OU and Uber (although it probably wouldn't do very well). However, if you have even
one Pokemon who's OU, then that team is above UU - you are no longer allowed to play in UU matches, only OU and Ubers. Breloom and Dragonite are both OU, therefore so is your team.
The tiers go basically in the order Grimdour posted - Uber, OU, UU, NU, and NFE. Not everyone considers Ubers a tier - it was originally created as a banlist for OU where people could stick Pokemon that were too powerful and unfair. For example, if you're battling someone with a good OU team and you whip out something like Kyogre, you're probably going to win unless they have something that's ridiculously specialized in order to beat Kyogre. Therefore, Kyogre is not allowed in OU, or anything lower than that. There's a similar tier in between OU and UU, called BL (or Borderline). It basically serves the same purpose, but it's a banlist for UU. Almost no one does BL battles, and it's my opinion that they could probably just stick BL within OU, but I understand that the OU tier is based on usage rather than ability.
Ubers consists mostly of the more powerful legendaries. As a general rule of thumb, if it's a significant legendary within the games, then it's probably Uber. Members of legendary trios (Articuno/Zapdos/Moltres, Entei/Raikou/Suicune, Regice/Regirock/Registeel, Uxie/Mesprit/Azelf) tend not to be Uber, and some even fall as low as the NU tier. Latias is not considered an Uber unless she has Soul Dew as a hold item, but Latios is an Uber in all cases. Shaymin is Uber if it's in its Sky forme, but not if it's in its regular forme. Phione isn't Uber, but Manaphy is. As well, not all Pokemon in Ubers are legendaries. Wobbuffet and Garchomp are both Ubers - Wobbuffet because its huge HP, its ability, and its moveset essentially guarantee that it will kill the opponent, and Garchomp because it completely upset OU and required people to form entire teams to counter it. It gets kinda confusing keeping track, so try looking up a tier list sometime.
Smogon's a good one, but if you can't get on Smogon, I can send you a private message listing all Ubers.
OU is also called the Standard tier, mainly because it's by far the most commonly-played tier. In fact, Pokemon are by definition Overused if they're among the top 50 most-used Pokemon on Shoddy. A few of the more powerful non-Uber legendaries are in it, as well as the majority of the strongest non-legendaries. As I said, your Dragonite and Breloom are both OU.
UU requires a bit more strategy because the people in it tend to be a little more skilled on average - this is probably because the majority of new players pick the strongest Pokemon they can and end up in OU. Your Clefable, Nidoking, and Kabutops are all UU.
NU... well, good luck finding someone to play NU with you. If something ends up here, it doesn't necessarily mean they're bad. It just means they'll need a hell of a strategy to work, and even then there's probably something that can do the job better and more easily. Your Ampharos is NU, which isn't exactly a good thing.
NFEs, for the most part, aren't worth considering. There are two exceptions that I can think of - Scyther, who works great in UU where Scizor is banned; and Chansey, who's basically Blissey for UU. It should be noted that if a Pokemon has something major over its evolution, it is placed in a higher tier (usually NU). For example, because Vigoroth doesn't get Truant whereas Slaking does, Vigoroth is in NU. Scyther and Chansey are both in UU because of usage; Scyther is faster than Scizor, and Chansey has only slightly worse special walling abilities than Blissey.
Okay, EVs stand for Effort Values. Effory Values are the number of Effort Points your Pokemon has from battling. This sounds as clear as mud now, but let me explain. The way the Pokemon games work, the stats you get when your Pokemon levels up is dependent on the things you kill. Every time you defeat another Pokemon, every Pokemon involved in that battle (even the ones that switched out before they did anything or the ones outside of battle holding an Exp. Share) gains a set amount of Effort Points (EPs) in a certain stat depending on what they beat. Let's use Zubat as an example; defeating Zubat gives 1 EP in Speed. So whenever your, let's say Nidoking defeats a Zubat, it will get 1 EP in Speed. You won't see the value anywhere in the game, but you'll definitely notice the results of EV training once your Pokemon levels up.
For every 4 EPs your Pokemon has in one stat, it will get an additional point in that stat when it is at its max level (100). So let's say your Nidoking would have 300 Speed at Lv100 without any Effort Points (which is bullshit, but bear with me). Having just 4 EPs will raise that Speed to a total of 301 by the time your Nidoking is Lv100. That doesn't sound like much, but you can have a maximum of 255 EPs in any one stat. That's a boost of 63 points to whatever stat you're training in. However, because stat boosts are only implemented in increments of 1 stat points for even 4 EPs, having 255 in that stat is pointless. So you round down to 252 for the best results.
Now, this doesn't mean you can create some super-godly 252-EVs-in-every-stat Pokemon. Your Pokemon can only have a grand total of 510 EVs altogether in all its stats. So that means you can pick two stats to put 252 points into, and one stat to put the remaining 6 EVs into. That's not always the best solution, however. It's best to either use Smogon to determine the best EV setup for your Pokemon, Google it, or (since you're already here) ask for help from people here or on other forums.
So okay, by the sounds of what I'm saying, you'll have to kill about 510 Pokemon to get the best EVs you can. If you're a normal person, you are probably currently thinking "fuck that noise" or something to that effect. But killing Zubats or Seedots or whatever isn't the fastest way, or the most recommended. First, you should find something that gives more EVs per kill. For example, Golbat gives 2 EVs instead of 1. That's still not a lot, granted, so let's look at hold items. The Macho Brace will halve your Pokemon's Speed, but double the number of EVs it gains per kill. So while holding that Macho Brace, your Pokemon will gain 4 EVs for every Golbat it kills. Now that's faster, but there's still something left to speed it up, although it's rare - the Pokerus. Every time you battle in the wild, your Pokemon has a chance of getting Pokerus. If you go heal your Pokemon at a Pokemon Center, the nurse will tell you about it. It appears as a status effect if you look at your Pokemon's details, and it wears off after a while leaving a smiley face on your Pokemon's summary window, but it's not harmful - on the contrary, it doubles the number of EVs your Pokemon gets per kill
again. So if you have a Pokemon with a Macho Brace and Pokerus, you're getting 8 EVs per Golbat kill. Damn, that's fast! And what's better, the Pokerus has a chance to spread to other Pokemon in your party (but only when it's shown as a status effect - when it's healed, as in with the smiley face, it'll still affect your Pokemon's EVs, it just won't spread).
Grimdour mentioned vitamins, so I will too - vitamins such as Carbos will boost your Pokemon's EVs by 10. They're not affected by Pokerus or Macho Braces, and they stop working once your Pokemon has over 100 EVs, but they can be nice to use.
If you want a bit more information, such as other EV-boosting items and locations on where to EV train, go
here.
IVs, like Grimdour said, affect stats as well. IV stands for Individual Value. Every Pokemon, upon being caught or hatched, is assigned an IV to each of its stats. The IV can be any number between 0 and 31. When your Pokemon hits Lv100, its stats will be higher depending on how high its IV in each stat is. For example, if your Lv100 Nidoking with 300 Speed had 0 IVs, the same Nidoking would have 331 Speed if it has 31 IVs. IVs can't be changed, but they can be influenced through breeding. Every time two parent Pokemon breed and a child Pokemon is formed, the child will take one random IV from each of its parents in a random stat.
Natures affect stats as well. Every nature gives a 10% boost to one stat and a 10% drop to another. As an example, let's use the nature Timid. Timid gives a 10% boost to your Pokemon's Speed, but a 10% drop in its Attack. So this is a great ability for something like Jolteon or Alakazam, something that relies on speed and uses special attacks instead of physical attacks. Some natures (Serious, Bashful, Hardy, Docile, and Quirky) give a 10% boost in one stat, followed by a 10% drop in the same stat, thus negating both the boost and the drop. These are called neutral natures.
A good rule in determining what boost or drop each nature gives is by finding out what kinds of Poffins your Pokemon likes or dislikes. The flavours below are matched to their corresponding stat:
Spicy = Attack
Dry = Special Attack
Sour = Defense
Bitter = Special Defense
Sweet = Speed
If a Pokemon likes one of these flavours, then their nature boosts the stat corresponding to that flavour; if a Pokemon dislikes one of these flavours, then their nature drops the stat corresponding to that flavour. So an Adamant Pokemon would like Spicy foods (therefore getting a boost in Attack) but dislike Dry foods (therefore getting a drop in Special Attack). Alternatively you can just look it up on the internet. o.o
If you play on ShoddyBattle, then you get to set all this stuff - natures, EVs, and IVs - when you create your team. I really hope you don't play Shoddy because god damn, wouldn't it suck to know I just wrote all of the above for no reason?
W/ Muscle Band A: Poison Point N: Sassy
W/ Life Orb A:Magic Guard N: Relaxed
W/ Expert Belt A: Battle Armor N:Adamant
W/Leftovers A: Inner Focus N:Brave
W/ ??? Item A:Static N: Bold
W/ Toxic Orb A: Poison Heal N: Careful
