Oxford is a university. Students belonging to Oxford have to go to its college. Different from Japan, University is not actually place you learn. It is just generic name of all colleges. So, when they meet the same university's students, they will ask first, "Which college are you from? it is interesting for us.
I will introduce the history briefly. it was build in 11th century. English was prohibited studying in Paris so it was made in England and it developed rapidly. That university has turned out 47 nobel prize winners, six Kings of England and more than 50 medalist.
more history here
There are 39 colleges. Teachers, students are belonging to one of them. According to the internet, there is a college accepting only women or graduate students.
There are unique customs of Oxford Unversity different from other Universties. For instance, there is a dinner that students must wear suit called "Formal Hall" and "Formal Hall" are classified with students status. Like this, there are interesting rules other Universities doesn't have.
