IV: Individual Value. All Pokemon have their own set of IV's for each stat, ranging from 0 to 31 that, along with EV's and Natures, determine how strong your Pokemon is in any given stat. You'll want IV's that are higher and as close to 31 as possible, or 31 if you're lucky or very dedicated to breeding.
EV's: Effort Value. Every Pokemon one of your Pokemon fights has EV's in a certain stat that your Pokemon gains when it defeats any given Pokemon (i.e., Bidoof gives 1 HP EV). The total EV's any Pokemon can have is 510, and the maximum EV's any one stat can have is 252 (every 4 EV = 1 stat growth). Once your Pokemon hits level 100, you can no longer manipulate EV's, as they're gained at level-up.