Groot is a Flora colossus from Planet X the capital of the branch worlds. The sapling that will come to be known as "Groot" came from an "Enobled Sap-line" and gifted in quasi-dimensional super-positional engineering.
Groot did not get along with his fellow saplings but instead preferred the company of the "Maintenance Mammals" who the other saplings treated with prejudice. After Groot killed another sapling to defend a maintenance mammal it was brutalising, he was exiled of the planet by the "Arbor Masters".
Invasion of Earth
For unknown reasons Groot attempted to kidnap a small town and bring them back to Planet X for study scientists of his home world. He appeared to be able to control most wood and attempted to use the trees to encase the town and trap the people, and appeared to be immune to the gunfire. He was eventually stopped by Leslie Evans who used termites to seemingly kill him. For unknown seemed to be able to efficiently communicate with the humans rather than them just hearing "I am Groot". 
Guardians of the Galaxy
Into his adulthood Groot found himself in Kree space, where he was promptly captured and imprisoned. While in jail he formed a rapport with Rocket Raccoon and was assigned to a covert ops team lead by Starlord, whose goal was to assault Hala and defeat the Phalanx. His body was mostly destroyed, but a small twig of him split off, preserving him, traveling in the care of Rocket Raccoon, fitting easily in his hand. He can grow back to his full size in time.
Groot and Rocket Raccoon are having a libation in a low-down dirty spacer bar in low-down dirty spacer space when they are accosted by a group of Badoon who attempt to arrest them for 'Crimes against the Royal Brotherhood'. A quick shootout ensues and Rocket creates an avenue of escape by shooting holes in the beer barrels causing the saloon patrons to stampede for the free beer. The duo make a hasty exit and are soon rocketing away when they find themselves pursued by Badoon Attack Cruisers. Rocket orders Groot to take evasive action while he goes to boost the stardrive to allow them to escape. He finds the stardrive compartment empty and the scene cuts to a shot of the surprised Rocket with a cliffhanger closing credits and Mojo proclaiming "Best Pilot Episode Ever!".
They then find themselves staring at an empty compartment where a stardrive should be and questioning why they are hearing voices like suddenly their lives just got a narrator or something. Their ship blows up and they appear in space in combat space armor bewildered at what just happened and how they got there. They target their Badoon attackers forcing one of the pilots to eject. They squeeze into his craft and it begins to plummet to an ice planet below. Mojo calls for the scene to cut and hit the Bio-Stasis on his stars. He proceeds to plan for the next scene expecting to make enormous money on the show. In the background, Rocket's Timely Inc. Shipment Processing Device analyzes the situation and informs the duo they are caught up in an artificial dramatic construction. They blast through a wall realizing they are in a "Flarkin' TV Studio" and are confronted by a hologram of Mojo who opens fire with real weapons.
Groot and Rocket Raccoon are transported to the Mojoverse by the inter-dimensional TV producer known as Mojo. He has decided to use the pair in his latest reality television show. He recruits various criminals that the heroes have fought in the past. He places the pair in scenarios were they would have to battle them again, however if an innocent life form was caught in the crossfire, they would be killed in the process. As the duo moved from one scenario to another, Mojo began advertising action figures with a collection pack that allows buyers to assemble their own 12" Groot figure after collecting all five. The demand was so high, Mojo stand to make a fortune until the Timely Inc. Shipment Processing and Analysis Device decides to take control of the situation and holds a B-00-M Meson Beam Gun at Mojo's face. It allowed the reality system to stop just long enough for the heroes to escape. They confront destroying Mojo who is revealed to be a robot controlled by Major Domo.
Groot, along with the rest of his team, aided the Avengers in fighting the returned Thanos.Thanos managed to get a Cosmic Cube from the United States Army, with which he escaped to Moord, the homeland of the Badoon. The Guardians arrived to the Avengers Tower and informed the Avengers about the situation. They joined the Guardians to fight Thanos and the Badoon.
After Thanos apparently killed the Elders of the Universe, to impose his supremacy, he became one with the Cosmic Cube and killed the Avengers as well as the Guardians. But actually, they were sent to the Cancerverse along the Elders, there, Tony Stark found that Thanos' weapon wasn't actually a Cosmic Cube and that it had defects. They bargained with the Collector, in exchange of a weapon capable of deactivating the "Cube" and return to Earth, the Avengers and the Guardians would let Thanos to be defeated by the Elders. With the help of other members of the Avengers, Thanos was defeated and sent to punishment by the Elders.
Soon after joining the Guardians on behalf of the Avengers as a liaison, Agent Venom started loosing control over the symbiote. After it was taken from him by the Guardians as a precaution, the symbiote escaped confinement and bonded with Groot.
Cellular regeneration: Groot has been seemingly killed on three occasions, each time regrowing from a sprig. This may be a major contributor to his botanopathy.
Strength: Groot's tree like form provides him great strength.
Durability: Groot's hide is composed of wood dense enough and durable enough to make him immune to most conventional weapons, including gunfire and fire which would consume most woods.
Control over other plants: Groot especially has the power to control other plants, or his own physical form, therefore he can turn his hands into blades, or grow in size altogether, or even use that power to heal himself from fatal wounds.
Genius Intellect: Groot has received the finest education from Photonic Knowledge of the Arbor Masters. He is highly intelligent and has a tremendous grasp of quasi-dimensional super-positional engineering.
"I am Groot!":Maximus the Mad, Black Bolt's brother, asserted that whenever Groot is saying his trademark "I am Groot!" he has actually been saying any number of things. It is the hardened nature of Groot's larynx that causes people, who are oblivious to the subtle nuances of his speech, to misinterpret him as merely repeating his name.
The Planet X which is home to Goom and Googam, as well as the Planet X of Kurrgo seen in Fantastic Four #7 -- later named Xanth in the Official Handbook to the Marvel Universe -- may or may not be the same planet X from which Groot claims to come. However, the Planet X of Fantastic Four fame has since been destroyed, while Groot recently returned home.
In Tales of Suspense #4, explorers from Earth encountered a seemingly uninhabited planet in system 427 in the year 2500. It turns out that the planet was actually dominated by sentient, animate trees. It is unknown if there is any connection between this race of trees and Groot.
Groot is also the dutch word for big, a fitting name for a hero that can increase his size.
In his first appearance Groot was seemingly able to communicate with humans beyond the usual "I am Groot".