EVs are Effort Values. They allow you to enhance the stats of your pokemon. You get them for defeating a pokemon. The number and type of EVs you get are detemined by the type of pokemon you defeat. Every 4 evs in a stat is equivalant to 1 point towards that stat. They can contribute to a higher hp, speed, attack, sp attack, defense, and special defense, all based on what the pokemon you defeat is. Ex: A bidoof yeilds 1 HP EV. If you kill 4 bidoofs, you will get an additional HP point overall. The most EVs towards a stat you can have is 255, but that isn't divisable by 4 (there would be 2 evs that wouldn't contribute to up the stat as 4 EVs= 1 point) so it's more practical to get 252 evs. You can get 510 EVs total. Certian items can add to(such as protien) / reduce (such as berries) the number of evs that you have in a particular stat. The pokevirus will give you double evs for each pokemon you kill.
See more: http://pokemondb.net/ev
IVs are individual values. They are randomly determinded when you meet a pokemon. There are 31 points that are distributed (randomly) amongst the stats. The more points, the higher that stat will be. The little discripton of your pokemon indicates which stat has the highest ivs. See more: http://pokemondb.net/mechanics/hidden
A pokemon's nature will add and subtract how many points go to different stats
Hope that was all clear enough.