Over a thousand years ago (10th centrury), the four greatest witches and wizards of the  same age shared a dream, to create a school where young people of the wizarding world could study and be trained in the magical arts they needed to  become skilled, fully-trained wizards. These four great wizards each founded one of the Houses ( Hufflepuff, Gryffindor, Slytherin, Ravenclaw)of the school that they worked together to form: Hogwarts School of Watchcraft and Wizardry.

For a number of years, this arrangement worked just fine. Each of the  founders took a group of students to be in their own house and even though they had differences of opinion about who should be trained, the house system helped them all to work together. However, as the years went by, the differences between Gryffindor and Slytherin became a serious rift.  Eventually Slytherin left the school.