The Kree are an alien race that exists in the Marvel universe. Hundred years ago the Kree visited the Earth, they sought to improve the human race and thus created the Inhumans as a result of their experimentation.
Marvel Cinematic Universe
Guardians of the Galaxy
Hundred years ago, the Kree and the Xandarians were involved in a war. This war was stopped when both races made a treaty to not attack each other. Despite the treaty, Ronan continued killing Xandarians, besides that Ronan made a deal with Thanos to destroy Xandar completely.
After getting the Orb, Ronan betrays Thanos and decides to stay with the Infinity Stone to destroy Xandar on their own, their plans are spoiled when he is defeated by the Guardians.
The G.H. is a kree that was used for the Project TAHITI to create drugs that are able to revive people. The drug created through fluids from the Kree was a able to revive Phil Coulson and save the lives of Skye.
In Shadows, it is revealed that the G.H. was owned by HYDRA during the World War II but was confiscated by the SSR when they arrested Reinhardt and his men. In the episode "The Things We Bury" Werner Reinhardt refers to the Kree as "Blue Angels" during the interrogation that Peggy Carter did to him.
They are revealed to be the ones who created the Inhumans as super weapons on various wars thousands of years pre-dating the pyramids.
Marvel Animated Universe
In The Avengers: Earth's Mightiest Heroes, the Sentry was sent by the Kree with the mission to destroy all defenses of Earth. When the Sentry attacks the observatory Ant-Man and Wasp try to stop the robot but fail in the attempt. After the attack of the Sentry, Captain Mar-Vel presents to the Avengers and informs them that the Kree are at war with another alien race, the Skrull, also informs them that the Sentry carries a Nega Bomb for special cases. However, the Sentry was defeated by the Avengers, after that a Kree Commander activates the Nega Bomb to destroy the Earth. Finally the bomb didn't explode thanks to the combined efforts of Iron Man, Thor and Captain Mar-Vell. Later, Mar-Vell informs to the Avengers that the Kree will return to Earth and that will not return to underestimate the people of the Earth. Ronan is sent to Earth by the Supreme Intelligence to see if humanity decides to join the Kree Empire and it is not, the Earth would be destroyed. Arriving at the Earth, Ronan confronts the Avengers and is defeated. After that, Ronan is confined on the special S.H.I.E.L.D. prison.
In the episode "Michael Korvac", is revealed that they were responsible for the kidnapping of Michael Korvac and give him his cosmics powers after his experiments.
Later, the Kree attempt to create a wormhole in the Sun, causing the end of life on Earth, the Avengers prepared to prevent this not happen are directed to stop. After the stop them, the Avengers fall in Hala, the home of the Kree, where they captured by the Kree troops.
Upon awakening, the Avengers are face to face with the Supreme Intelligence, the Supreme Intelligence is willing to kill them but the Captain Marvel speaks about the abilities of the Avengers and would be useful for the Empire. The Supreme Intelligence agrees and orders to dissect the Avengers, meanwhile on Earth, Ronan is freed and recovers its Universal Weapon.
Later, the Supreme Intelligence is defeated by Captain Marvel, who is in charge of the Kree Empire.