"I am Groot" - Groot, all the time, for every occasion.
Groot is a Flora Colossus, a species of plant-like humanoid organism, native to the Planet X. His ability to communicate is limited to only those who can understand his esoteric vocabulary. To the rest of the universe, whatever he says pretty much sounds like "I", "am", and "Groot", and in that order. Groot is the loyal sidekick, friend, and partner in crime of Rocket Racoon, who is a wanted criminal but isn't really all that bad. Rocket is one of the few who knows and fully understands the esoteric vocabulary. Somewhere in their timeline, Groot and Rocket get entangled with Star Lord and Gamora during one of their crime missions, and after a couple of battles against and with each other, they finally realize that they are on the same side., The four form an alliance that would become the Guardians of the Galaxy (GOTG). The GOTG are believed to have also eventually learned Groot's language and are able to communicate with him to some extent.
Flora Colossus are described as powerful and tough humanoids that is almost invulnerable to physical harm. They have rapid healing and regeneration powers, and are able to grow limbs and body parts (even their entire form) from nothing more than a twig. They have full control of their regeneration and are able to grow their limbs to take various shapes to be used as weapons or shields, and can even grow larger at will. Groot is the last surviving member of his kind. As seen in the GOTG film, Groot respawns as a baby version of himself if and when he dies. Baby Groot becomes a fan-favorite very quickly because of his cute demeanor.
In the original version of Groot's story, he is actually an alien who comes to Earth to capture humans and study them. However, he is defeated and almost completely destroyed when Xemnu, an intergalactic criminal makes a duplicate version of him by combining a human and a tree to fight Hulk. The duplicate is defeated, and it is later revealed that Groot has not died. Groot is then captured and kept captive by The Collector in his zoo before he is freed and together with other monsters rampages New York City. He is later tracked down and captured by S.H.I.E.L.D. In this version, Groot does not join the GOTG.
Among Groot's abilities are:
Super strength - He is strong enough to lift very heavy objects effortlessly and his limbs are stronger than most hard materials including metals.
Durability - His robust skin and physique is hard to break or penetrate. He is able to sustain high doses of physical damage.
Speed and agility - Despite being quite large as a fully grown Flora Colossus, he is still considerably fast and agile.
Body manipulation - Groot can manipulate his limbs and body to take various shapes. He can grow vine tentacles, generate wooden spikes, create a shield, and even emit bioluminescent spores that give light.
Rapid regeneration - His limbs and body can regenerate as rapid as they are destroyed by physical damage, making him really, really hard to kill.