Oxford, Cambridge, Durham and a few other universities have a system based on 'colleges' (originally groups of scholars, who lived together and organized study and - sometimes - teaching). Different colleges were founded at different times. In the very early days, the colleges weren't even part of the university (colleges came together and said 'lets organize ourselves as one institution, while keeping individual responsibility for some things). See Collegiate universities - Wikipedia, the free encyclopedia
Any student at Cambridge University belongs to a college; when I said 'my college' I meant 'the college I lived in when I was a student at Cambridge, and to which I still belong'.