The Guardians of the Galaxy were a band of former intergalactic outlaws, who have teamed together to protect the Galaxy from planetary threats. They managed to save the planet of Xandar from the Kree warlord Ronan and have been recognised under the Nova Empire.
An undetermined time after, they defeated the Abilisk, attracted the unwelcome attentions of Ayesha and her Sovereignrace, and defeated the Celestial Ego.
Four years after that, the Guardians joined forces with the Avengers to stop Thanos from collecting the Infinity Stones.
Gamora was the first Guardian to be killed when Thanos sacrificed her for the Soul Stone. Despite their best efforts to stop Thanos when the majority of them faced him in battle on Titan alongside Iron Man and Spider-Man, they were defeated and Thanos then arrived on Wakanda and defeated Groot and the other heroes effortlessly and completed the Infinity Gauntlet.
The only survivors among the Guardians were Rocket Raccoon and Nebula as all the other Guardians were erased from existence when Thanos used the stones to kill half of the universe's population.