Essentially, every single pokemon of the same species has identical stats to each other. However, each pokemon when you first encounter it or hatch it is assigned a secret, invisible number called IVs that range from 0 - 31 which, by the time it reaches level 100, will mean that pokemon gets 0 - 31 extra points in each stat. (so if you have a pokemon with 31 IVs in every stat, that pokemon will be 31 points stronger in every single stat that a pokemon who has 0 IVs in each stat)
You cannot change, alter or have any control over what IVs get in which stat, they are predetermined from the moment you encounter the pokemon. You can, however, pass on IVs from parents onto children via breeding and use of an item called the Destiny Knot. (breeding is a whole other topic!) so semi strong parents can create super strong offspring.
EVs on the other hand, are invisible numbers given by every pokemon you battle (so every Zigzagoon you fight gives you 1 point in speed, every Scatterbug you fight gives you 1 point in Defense, and so on) and for every 4 Evs you get in a particular stat you get an extra point in that stat (so every 4 Scatterbugs you fight you get 1 extra point in Defense) up to a maximum of 252 EV points which translate to 63 extra points to a stat by the time you get to level 100. in total, a Pokemon with perfect IV and maxed EV training in a stat will be 94 points stronger than the identical pokemon with 0 IVs and no EV training in that stat. Now, even if you have perfect IVs and maxed EVs in a stat, you won't necessarily see these points until level 100. They are still there, but at lower levels the gain isn't as apparent as it is in higher levels.
Hopefully that makes it clearer.