The two most common usages of “college” in UK education are:

  - pre-university education for 16-18 year olds. This is where you complete qualifications for university entry, such as A Levels, Scottish Highers, or the IB, as well as more vocationally aligned qualifications like BTEC or NVQ. This stage of education used to be optional but recently became compulsory.
  - institutions within a larger university umbrella. This includes the colleges of Oxford, Cambridge and Durham. For example, Jesus College of the University of Oxford or Magdalene College of the University of Cambridge. Very few UK universities use a college system, it is mostly a historical legacy seen in very old universities.

Another reply mentions Imperial College London: many of the London universities are colleges of the University of London, but I think this system is mostly so they can share/issue one University of London degree, and that they don’t have the same degree of centralised administration that the Oxbridge college system does.

EDIT: updated to reflect that A levels aren’t the only pre-university qualification in the UK