An A Level Maths course typically takes two years (years 12-13). A student who only completes the first year of a full A Level will obtain an AS Level qualification. A student who completes both years will obtain a A Level qualification (AS results to not carry over). AS in itself is not enough of a qualification for university entrance.
The following exam boards are available for A Level maths:
Edexcel (a UK and also an international IAL exam board)
Cambridge (UK and international)
CCEA (Northern Ireland)
A student will study the exam board that their school has chosen. The order of difficulty from least to greatest is AQA, Edexcel, OCR and Cambridge. Edexcel is the most common exam board offered. Universities do not favour specific exam boards. The exam board one takes is a matter of choice of each school, and won't make any difference to ones chances of being accepted into university. Some schools even use different exam boards for different subjects.
The content covered is now the same across all UK exam boards (AQA, Edexcel, OCR and OCR MEI). It is only the structure of the exam that differs. In contrast, international versions of the A Level exam are offered by CIE, Edexcel IAL and Oxford AQA. These international versions differ in content AND structure, unlike the UK versions. Edexcel IAL and Cambridge CIE exams are moduluar which means one can complete individual units at different times throughout their course, thus allowing students to take exams at the right time for them for them. Students can resit individual units due to the modular nature of these qualifications. This is in direct contrast to the UK where all exams for AS and A level are linear, which means they are are taken at the end of the course (in the second year of study).
A Level Further Maths is a totally separate A Level to A Level Maths. One won't be able to take Further Maths unless you are also concurrently taking A Level Maths or already have a Maths A Level.
A Level Maths past papers, questions organised by topic, cheat sheets, condensed revision notes, general advice, solution banks & textbooks are available below for all exam boards - Edexcel, Edexcel, AQA, OCR, WJEC and Cambridge.
Grade boundaries fluctuate between different exam boards. See grade boundaries for the breakdown for each exam board and paper.