They're terms for how a Pokemon's stats will increase as you train. xD Like. IVs are what a Pokemon is born with, so you can only really hope to get good ones upon receiving the Pokemon.. and EVs are earned from defeating other Pokemon, depending on which species you battle. A Pokemon high in Special Attack, perhaps a psychic-type, would probably give you points in Special Attack, while a fighting-type would lean toward Attack points.

A good example of a Pokemon you'd want to control EVs on is Tyrogue. Since what it evolves into depends upon its defense and strength stats, you'll wanna make sure you battle the right Pokemon to influence them so those stats will increase the way you want 'em to. Especially if you want a Hitmontop.

'Cause I swear, I went through like, three Tyrogue trying to get one because I was just relying on luck. Then I just gave up and fed one stat-boosting items to bump the lower stat up.

And of course, you may just want to get good EVs so you end up with better stats where you want 'em. It may sacrifice points in the stats you're not EV-training for, but those that do get the points will certainly be higher.

I couldn't put it into better terms than Yoshi did there. xD