Hagrid is as much as twice as tall as the average man. He loves magical creatures, particularly unusual or even dangerous ones. J. K. Rowling has said that Hagrid has little interest in tame magical creatures because of the lack of a challenge. He befriends Harry Potter and his friends and looks out for them. They do the same for Hagrid.
Hagrid was a student at Hogwarts but was expelled during his third year after an incident involving the Chamber of Secrets. Dumbledore nonetheless gave Hagrid the job of looking after the grounds (especially the magical animals). Since Hagrid did not complete his training, he was forbidden to practice magic, though this ban was lifted when he became a teacher.
Hagrid rescued Harry from his parents house, which was destroyed by Lord Voldemort, and in Harry Potter and the Philosopher's Stone he is given the task of locating Harry, as Vernon Dursley continually attempts to escape Harry's school letters. He helps Harry to find his bearings in the magical world, as well as helping him to buy his school things.
Although not a schoolmaster in Harry Potter and the Philosopher's Stone, Hagrid does have a measure of authority. When Draco Malfoy balked at completing a detention, Hagrid threatened him with immediate expulsion: "You'll do sommat useful, or you'll get out . . . go pack your bags." This could, of course, be that Hagrid knows that he will be expelled by others, because he knows the rules.
Hagrid's love of dangerous magical creatures is central to the plot of Harry Potter and the Philosopher's Stone: a hooded person (Professor Quirrell in disguise) gives him a dragon egg to elicit details about Fluffy, the three-headed dog Hagrid has lent to Dumbledore to help guard the philosopher's stone. Hagrid again lets slip, to Harry, Ron and Hermione, about how to get past Fluffy (by playing music), which allows them to pursue the potential thief. The three also assist Hagrid after the dragon egg hatches, by helping to remove the baby dragon Norbert, who is taken to live with other dragons by friends of Charlie Weasley.
In Harry Potter and the Chamber of Secrets, Hagrid is sent to the wizards prison, Azkaban, when the Chamber of Secrets is opened because of his prior expulsion. He is later exonerated when the real culprit is found, and released. He helps Harry and Ron in their quest to solve the mystery of the Chamber of Secrets by telling them to follow the spiders—leading them to Aragog, the giant spider blamed for the original attacks, which Hagrid had kept as a pet while a student at Hogwarts.
In Harry Potter and the Prisoner of Azkaban, Hagrid takes over the Care of Magical Creatures class. Draco Malfoy causes trouble in Hagrid's class by insulting a Hippogriff (which then attacks him for the slight), and the Ministry of Magic issues an order for the beast's destruction, much to Hagrid's distress. Harry and Hermione Granger save the Hippogriff by going back in time.
It is also revealed, Harry Potter and the Goblet of Fire, that Hagrid is of mixed human and giant parentage. Since giants have a reputation for being horribly brutal, and were once allies of Lord Voldemort, Hagrid keeps his parentage secret and allows people to imagine other reasons for his great size. The book ends with Hagrid setting out, with Madame Maxime—a half-giantess with whom he has become romantically involved, as an envoy to the giants.
In Harry Potter and the Order of the Phoenix we find out that though his mission, despite early promise, was unsuccesful—though he does return with his half-brother, Grawp. Harry and Hermione agree to take care of Grawp in Hagrid's absence—Hagrid fears (correctly) that he is to be driven out by Dolores Umbridge. Grawp later helps Harry and Hermione, when they are under attack by centaurs.
In Harry Potter and the Chamber of Secrets, the date of that particular school year is firmly established as being 1992-1993. It also says that Hagrid was expelled 50 years prior to this (1942), during his third year at Hogwarts. A third year student is 13 years old making Hagrid's year of birth approximately 1929.