In Liquestria you play as one of five races: Earth Pony, Unicorn, Pegasus, Zebra, or Griffon. All five can do just about everything you can do in vanilla Minecraft, but they each have their own unique skills and abilities on top of that which offer some interesting quirks and gameplay mechanics.
All of the races have their own traits (Earth Ponies are more hardy and athletic, Pegasi can fly) and their own particular set of abilities (Unicorns can cast various kinds of spells, Griffons are master archers who can launch volleys of arrows).
When you apply to become a member of Liquestria, you are asked to choose between one of the five. You can change your race later until you find one you really like. We would prefer for people to eventually stick with one race, though, so once you have some play time, give it some thought.