Effort Values, more commonly known as the abbreviation, "EVs," are some of the most complex subjects in the topic of Pokemon. I'm sure you know about stats? HP, Attack, Defense, Special Attack, Special Defense and Speed? Well, in years ago, a group of scientists (AKA PokeNerds) discovered EVs. 252 of the little things can be placed in two stats and 4 in another. Here's what they do ingame and competitively.
EVs naturally come to your Pokemon as you battle with them. Each Pokemon has an EV yield of a number and stat. For example, Beautifly has a Special Attack yield of 3. That doesn't mean if you're using a Beautifly that you will get 3 Special Attack EVs, but if you happen to battle one and defeat it, your Pokemon's Special Attack stat will go up by three EV points. One more and the stat will go up by 1!
As soon as word of EVs reached the competitive side of the battling universe, heavy battlers used it to their advantage. Often we make EV spreads, which every competitive battler knows about.
Individual Values, more commonly known as IVs, function similar to the manner of human features. They come at birth (AKA Pokemon hatching from an egg), and stay unless you do some plastic surgery (AKA Action Replay).
They raise your stats. Maximum raising is 31. Say your Speed is 100. With 1 IV (Minimum), the stat will become 101. But with the maximum 31 IVs, it will become 131. Unlike EVs, all stats can have maximum IVs. The standard IV spread in competitive is 31 for all of them.