Starfleet is a fictional organization in the television and film franchise Star Trek. Within this fictional universe it is a deep-space exploratory, peacekeeping and military service maintained by the United Federation of Planets ("the Federation"). It is the principal means by which the Federation conducts its exploration, defense, diplomacy and research. While the majority of its members are human and it is headquartered on Earth, Starfleet is composed of hundreds of species (which includes the majority of the shows' protagonists).
Read more about Starfleet: History, Mission, Different Species in Starfleet