This seems to still be a contenious issue for many people so it is probably time for an update as o what everything is and the implications.
Triple science GCSE - the most to easy understand. 3 separate GCSEs in biology, chemistry and physics with the grades being individual for each. Lots of schools now start this in Y9 because it doesn't always get a decent time allocation in KS4. Leads onto A-levels in biology, chem, phys etc (usually criteria is a B in the subject for progression to A-level) as well as the other post-16 options like A-level applied science, BTEC level 3. Students achieving L6 at KS3 should be given the opportunity to study triple sciences but these are only recommendations and not legally enforceable.
Double award options..........
Science + additional science - A traditional course combining biology, chemistry, physics but with the equivalent modules of 2 GCSEs. Grades awarded separately so can be different for the science part and the additional science. Progresses to all A-level and BTEC level 3 routes but for A-levels in pure sciences the college looks for a B in the additional science component usually.
Science + applied science - A general science course with the second award being more based on work related contexts. The 2nd GCSE the 'applied' GCSE usually has more coursework than a traditional course (approximately 50%). Rarely accepted as suitable for progression to pure science A-levels but can progress to A-level applied science (which isn't a bad course by the way, very good for potential nurses, primary teachers who just want a bit more science) or BTEC level 3.
Double award applied - Same as above but both awards are in a workplace context and have a larger coursework content. Same progression as above.
Single award.........
Many schools will enter lower ability students for the compulsory core science GCSE but some will just automatically use the BTEC route. No progression to A-level pure sciences, some colleges will accept for A-level applied science if they get a C or BTEC level 3.
BTEC - 100% coursework assessed. Can be equivalent to 1 or 2 GCSEs at level 2, pass, merit or distinction (equivalent to C, B, A GCSEs). No progression to A-levels in pure sciences, possible progression to A-level applied science in some colleges, usual progression is to BTEC level 3 but this is not commonly accepted by universities as suitable for entry. My word of caution in this is that it is not particularly respected as currently all work is assessed internally and there have been cases of students gaining credit where work does not exist, is copied or even done in a week. This has all weakened the position of BTEC although there are moves to strengthen the external assessment. It isn't accepted for progression as the mode of delivery in many schools means students retain very little understanding of the science after they have completed the task required to get the mark. Most schools use it as a way of getting a C equivalent double award out of students who wouldn't get that at GCSE although this is likely to change very soon as it isn't counted in EBacc and we are still awaiting the decision on the double science quality indicator.
OCR nationals - v similar to BTEC.
A few other things you might be bombarded with by schools..........
Level 1 qualification - Equivalent to GCSE grades G-D
Level 2 qualification - Equivalent to GCSE grades C-A*
Level 3 qualification - Equivalent to A-level grades E-A*
Hope this helps a few people.