Currently, there are 4 free Warlord skins (Adam, Son of Woods, Trixy, and Void Spirit). As long as you are willing to pay you can unlock skins that will be similar to one of the 4 free skins. While these skins are similar, they will each come with their own abilities which might be a slightly boosted version of a main skin, or a combination that is almost completely different. Each skin's abilities unlock once you reach a certain Warlord rank with that skin. Once that rank is reached the ability will be available after a certain number of turns have passed. The first ability is available at the start of each battle, while the second one is usually available on or after the 3rd turn. The last ability becomes available on or after the 7th turn. You may use them whenever they are needed as long as they are available.
Different skins will be more effective in different situations; this will depend on what your overall squad strategy is. Freebooter Dave is considered by many to be the best early game skin that you can get, while Og'ord, Crusher is considered the best late game skin. As you play, you will find which skin works best with the heroes that you have.