What is an open university ?

The Open University is based in the UK, that offers distance and Adult courses.
Distance Learning Courses and Adult Education - The Open University

There are loads of 'Open Universities' here is a list of them;
Category:Open Universities - Wikipedia, the free encyclopedia

And here is a good explanation;
Distance education - Wikipedia, the free encyclopedia

I study through the UK based one, basically it is a distance learning University where you pay to be enrolled on courses, they send you the study material and you work through it at home, sending in assessments and assignments to gain your grades.

With the UK based one you can study right up to Masters in many subjects.