Oxford University is made up of more than 30 different colleges. Colleges are academic communities, where students usually have their tutorials. Each one has its own dining hall, bar, common room and library, and lots of college groups and societies.
All undergraduate students at Oxford become members of a particular college, and of the whole University.
Colleges accepting undergraduate students
Colleges do not specialise in particular subjects as most colleges offer most courses and excel in all the subjects that they teach. (See Which Oxford colleges offer my course?)
Your lectures, assessment, and any practical work will be organised by the academic department(s) who run your course, while your college will organise your tutorials and provide you with accommodation and other facilities.