IVs (individual values) are numbers that pokemon get for each stat. The number is decided when they are encountered in the wild or hatched and cannot be changed, ever. Basically, the higher the IV, the better a pokemon performs in that stat.
EVs (effort values) are a bit like IVs in that they affect a pokemon's performance in a certain stat, but the difference is that they can be changed. Pokemon in the wild give out EVs when you battle them, and each one gives out a certain EV in a certain number. Every four EVs you get in a stat translates to about one stat point at level 100, but they will still raise your stats before you get to that level. Basically, any EVs you get will increase the pokemon's stats when they level up. You can have a total of 255 EVs in any one stat, but that's not divisible by four, so the extra 3 EVs won't make a difference. That's why most people only raise their EVs to 252. In total, you can have 510 EVs, so if you raise two stats to 252 EVs, there are still 4 left over which can be given to any stat you desire.
I hope that helped.