Read the official story of the Hogwarts Express on Pottermore.
Houses:
Hogwarts is divided into four houses, each bearing the last name of its founder: Godric Gryffindor, Salazar Slytherin, Rowena Ravenclaw and Helga Hufflepuff. Throughout the school year, the houses compete for the House Cup, gaining and losing points based on actions such as performance in class and rule violations. The house with the highest end-of-year total wins and has its colours displayed in the Great Hall for the following school year. Each house also has its own Quidditch team that competes for the Quidditch Cup. These two competitions breed rivalries between the houses. Houses at Hogwarts are living and learning communities for their students. Each house is under the authority of one of the Hogwarts staff members. The Heads of the houses, as they are called, are in charge of giving their students important information, dealing with matters of severe punishment, and responding to emergencies in their houses, among other things. Each year, year level groups of every separate house share the same dormitory and classes. The dormitory and common room of a House are, barring rare exceptions, inaccessible to students belonging to other Houses.
In the early days of Hogwarts, the four founders hand-picked students for their Houses. When the founders worried how students would be selected after their deaths, Godric Gryffindor took his hat off and they each added knowledge to it, allowing the Sorting Hat to choose the students by judging each student’s qualities and placing them in the most appropriate house. The student’s own choices may affect the decision: the clearest example is the Hat telling Harry that he would do well in Slytherin in the first book, but ultimately selecting Gryffindor after Harry asks it not to put him in Slytherin.
See Pottermore for the full story of Godric Gryffindor.
Gryffindor House:
Gryffindor is one of the four Houses of Hogwarts School of Witchcraft and Wizardry and was founded by Godric Gryffindor. Godric instructed the Sorting Hat to choose students possessing characteristics he most valued, such as courage, chivalry, and determination, to be Sorted into his house. The emblematic animal is a lion, and its colours are scarlet and gold. Sir Nicholas de Mimsy-Porpington, also known as “Nearly Headless Nick” is the House ghost.
Gryffindor corresponds roughly to the element of fire, and it is for this reason that the colours scarlet and gold were chosen to represent the house. The colour of fire corresponds to that of a lion as well, with scarlet representing the mane and tail and gold representing the coat.
Traits:
The Gryffindor house emphasises the traits of courage as well as “daring, nerve, and chivalry,” and thus its members are generally regarded as brave, though sometimes to the point of recklessness. Some Gryffindors have also been noted to be short-tempered. Notably, Gryffindor house contributed many members to Dumbledore’s Army and the Order of the Phoenix, although this may have been because the main members made it a point not to associate with other houses.