Like primates, marsupials are a type of mammal. One thing all members of this family have in common is a pouch.
The most famous marsupial is the kangaroo, but there are many others, such as wallabies, opossums, koalas, and wombats. What makes marsupials different from primates or rodents (who are also mammals) is that the mothers have pouches to hold their young. This is because when marsupial babies are born, they're not quite ready for the world, so the pouch gives them a chance to grow and be safe before having to live on their own. When you think marsupial, think "pouch."