OWLS - O levels which became GCSEs in 1988. These are the public exams one takes at 16+. You can leave school without them, but you need a minimum of 5, including Maths and English, to get into university. NEWTS - A levels. These are taken two years later, so at 17/18. They have changed, recently, to become mostly modular courses with minor exams halfway. The O stood for Ordinary (those too 'thick' to take O levels took CSEs, incidentally; now everyone takes GCSEs but there are different levels of paper; you can only get an A by taking the highest level); the A stands for Advanced.

There are other qualifications you can take while at school, sixth form college, or other Further Education establishment, e.g. City and Guilds (vocational qualification, sometimes regarded as equivalent to 2 A levels); the International Baccalaureate (broader base of subjects, more comparable to a US High School diploma); OND and HND BTEC qualifications, RSAs... all but IB are vocational though, so you would rarely need to know more about them.

I have: O levels, A levels, GCSEs, an RSA, a City and Guilds, some of which I did at technical college rather than school. I was just the right age to slip in an O level before they became obsolete - the current GCSE cohort were born after O levels were abandoned. Now I feel old.