EV's and IV's are your friends.

[SHS_Smeth]

IV's
EV's  

[dragwarrior]

im srry for liking pokemon and not knowing what these mean but what does EV and IV mean?  

[iSephi]

((In response to Dragwarrior though many could benefit))

IV:Individual Value: The certain value given to each of your pokemon's stats, from [0] to 31 with 31 being the best, these influence what type of pokemon it would be Sweeper Wall Etc
[IVs are somewhat like genes, since they're determined when you find them in the wild or hatch them from an egg and they cannot be changed after that point. For every point increase in your Pokemon's IV, its corresponding stat will be 1 point higher at level 100. ]

EV:Extra Value/Effort Value: These are awarded every time you kill a pokemon, it awards up to 3 points, and using the related band/lens/anklet gives you an extra 4 IV's Per pokemon Killed.
[EVs are values from 0 to 255, and for each Pokemon a maximum of 510 can be obtained. At level 100, your Pokemon's stats will be 1 point higher for every 4 point increases in EVs, rounded down, for a maximum of 63 points per stat. As such, 127 points can be added to stats this way.]

[Some clarification/content edits by darkslasherX, for more info read the guides above.]