Kerrigan will be a playable unit in most missions of StarCraft II: Heart of the Swarm. When the former Queen of Blades takes the field, she has multiple roles to choose from. Each role is called a Battle Focus and comes with a a unique set of abilities. As the campaign progresses, new roles will unlock, and Kerrigan will acquire new abilities for existing roles. The Battle Focus you choose for Kerrigan in any given mission and the way you use her powers will have a major impact on how each mission plays out.
Harder, better, faster, stronger: the Swarm never stops evolving, and in Heart of the Swarm, you decide how the zerg develop. Between missions, you'll be able to mutate your different zerg species. If you unlock enough mutations for a species, you can evolve that species into one of two different strains. For example, one zergling strain might evolve the ability to hatch more zerglings from the same egg, while the other could produce zerglings that can make short leaps forward to close into melee distance more quickly.