A-level: Courses have an end-of-year exam (up to three) and occasionally coursework. You generally take 4 AS levels (sorta half a-levels) and carry three of them onto the full A-level
BTEC: Can be taken at Level 2 (GCSE) or Level 3 (A-level) standard. No exams, all coursework. Often taken instead of an A-level (or GCSE). Not as respected as A-levels.
Diploma: I think in the context you want to know, it's just a kinda of BTEC. BTECs can have different amounts of modules attached to them, giving them more/less weight, and a Diploma is just one of these classifications.