But I thought Pokémon was easy... Welcome to level five.
A level five trainer gains a basic understanding of Effort Values (EVs). This includes utilizing vitamins, excluding the use of Rare Candies, and not training against limited opponents.
Effort Values are one of, if not the most complex aspects of Pokémon. I'll go more in depth later. In the mean time, take a brief summary. EVs are what a Pokémon gains when they defeat another Pokémon (Besides Exp). Every single Pokémon has what's called an EV yield. When they're defeated, the EVs are added to their defeater's stats. However, every EV only counts for a fraction of a point in the stat the EV counts towards.
In the Pokémon world, their are items called Vitamins. HP Up adds HP EVs, Protein adds Attack, Iron increases Defense, Calcium increases Special Attack, Zinc increases Special Defense, and Carbos adds Speed. There is a limit to how many of each that can be used, as well as a limit to the total amount of vitamins.
The Rare Candy is a single-use item that, when used, immediately levels up the Pokémon it was used on. This item may seem infinitely useful to a level one trainer, but at this point, its use is not encouraged. The reason being that leveling a Pokémon up with Rare Candies causes them to be elevated devoid of any EVs. They can, however, be used without remorse if their subject has had its EVs maxed out.
Pokémon Black and White added items called Wings. The Wings have a lower EV yield than Vitamins, but they can go beyond the limit to which Vitamins are bound. Every Pokémon can only hold a certain amount of total and individual EVs. Wings can reach this limit, but training off of opponents' EV yields is more well worth it. If nothing else, it's much faster.
When training, trainers often use a tactic called Level Grinding, a strategy in which the trainer will fight the same Pokémon repeatedly. While this tactic is effective for leveling up, it often makes for an ineffective spread of EVs. Unless the opponents are carefully selected. (More on this particular method later.)