The Guardians of the Galaxy (GOTG) are a group of misfits that band together to form one of the most prominent superhero groups to appear on the Marvel Cinematic Universe (MCU) timeline. They were introduced in MCU's Phase Two as the gang who would be responsible for keeping the Power Stone away from Thanos.
It all starts with a half human-half celestial being named Peter Quill who has never known his father, as his parents were separated as far as he is able to remember. On her dying bed, Meredith, Quill's mother, describes his father to him and later in his adulthood, he would learn that his father was a Celestial Being named Ego. Upon the death of his mother, Quill is abducted and taken on board an alien pirate spaceship belonging to the Ravagers, led by the pirate Yondu Udonta. Fast forward 26 years and Quill leaves the Ravagers and tries to screw them over by stealing an orb (which the GOTG learn contains the Power Stone) and tries to sell it to someone called The Broker. At this point, solo Quill names himself Star-Lord.
During his mission to sell the orb, he crosses paths with Gamora and Nebula, the daughters of Thanos, who have beensent to also secure the orb together with Ronan, and Rocket Racoon and Groot, the unassuming bounty hunters and partners in crime. They get engaged in a battle with one another, only to be captured and imprisoned by The Nova Corp. During this time, Gamora plans to betray Ronan and her mission, leaving Star-Lord on his own side and Rocket and Groot on the third front just interested in making money.
While in prison, they meet Drax the Destroyer, the alien warrior who only wants to avenge the death of his family at the hands of Ronan. Together, they come up with a plan to escape from prison, secure the orb, and then battle Ronan. At the end of the battle, they use the power of the orb together to destroy Ronan and take responsibility for the Power Stone. Thus, is born ,the Guardians of the Galaxy.
In Phase 3 of the MCU, the GOTG manage to recruit two more members, Mantis and Nebula, who play a very significant role in helping the Avengers defeat Thanos and his army. In the final phase and after the defeat of Thanos, they have one of the most powerful characters in the universe join their gang - Thor, the God of Thunder.