In the UK, at sixth from (age 16 to 18, after GCSE) you do your A levels. The first year of these is called the AS levels, which count for half the A level. Then the second year is called A2 which counts for the other half. Typically you will do 4 AS levels, then drop one and do 3 A2s.
After A levels, you can go to university, which is where you get your degree. Most degrees take 3 years (though some take more), and whilst you do this you are an "undergraduate". To get into Cambridge you need A levels (usually 3 at grade A), although you will apply after you just have your AS levels.