The greater majority of cards can be evolved. Cards chosen for evolution must share the same name; evolving will increase their stats.
Cards that are evolved grow stronger, but the resulting card only retains 5% of the parameters of each of the material cards, meaning it is wise to enhance your material cards before evolving them in order to maximize the added benefit for evolving.
When you evolve a card, its overall level and skill level will revert back to level one, meaning you'll have to dedicate more time to enhancing the evolved card.
The progress of each card is displayed in the top right hand corner. Some cards require a specific item or material card in order to evolve. In this case, the card cannot evolve with other cards of the same name.