IVs are short for Individual Values.
>But what does that actually mean?
Unlike EVs (Effort Values), IVs never change over time. They remain the same ever since you caught it or got it as an egg from the Day Care center. The game randomly assigns a number from 0 to 31 to each of its stats (HP / Atk / Def / Sp. Atk / Sp. Def / Speed). If you're breeding, you can manipulate these IVs (see here). I'm getting side-tracked here though. Anyways, these IVs determine whether the Pokemon would yield higher stats. For example, 31 IVs would yield higher stats, while 0 IVs would yield lower stats.
You can read more here: IV Guide
When you are talking about 252 Sp. Atk or 6 HP, you are then talking about EVs. As I mentioned earlier, EVs are Effort Values. These EVs can boost your stats and you can gain them by defeating wild Pokemon or doing Super Training (which is only available in XY and ORAS). Each stat can have a maximum of 252 EVs invested in it. The total amount you are allowed is 510. So this means that you can max out in two different stats and put 4 EVs in another. Now, you're probably wondering why there is 6 HP. There can be an extra 2 EVs received somewhere ( I think it was in BW ), but this extra 2 EVs doesn't really matter as it does not boost a stat. If the Pokemon is level 100, the EVs work in groups of 4 (each group of 4 gives one point in your stat).
You can read more here: EV Guide
>And also what are sweepers, walls, and tanks are and how to tell them apart?
Since I can write 3 whole paragraphs describing about it (but I bet you're tired by now), I'll just quickly explain it.
Sweeper - A Pokemon that uses Offensive moves to bring a team down. They are generally categorized by their high speed and their high offenses (Sp. Atk and / or Atk). There are many different kinds of sweepers, such as set-up sweepers that use stat-boosts (ex: Dragonite may set-up D-Dance), mixed sweepers that use both Atk and Sp. Atk, physical sweepers that use their Attack, and special sweepers that use their Sp. Atk.
Wall - A Pokemon that tries to "wall" moves and take physical and / or special attacks extremely well and consistently over a course of the battle. They are generally categorized by their high Defenses / Sp. Defenses / HP.
Tanks - This is sorta like Walls, but they can deal damage back to the opponent. They also do not have to do it consistently over a battle.
Here's where you can read more: Pokemon Dictionary (it may be outdated, but it should still work).
Anyways, that's it! I hope I helped!