UK Special Forces, explained plainly
United Kingdom Special Forces is the directorate that brings the country's special forces units together. It includes the SAS — 22 SAS regular, 21 SAS and 23 SAS reserve — and the Special Boat Service, the Royal Navy's special forces. Civilians with careers apply to the reserve units; the regular units recruit from serving personnel.
The SAS
22 SAS is the regular Army regiment. 21 SAS and 23 SAS are its reserve counterparts, recruited regionally from people with civilian jobs. If you are reading this while employed and wondering whether it is possible, the reserve units are the route you are asking about.
The SBS
The Special Boat Service is the Royal Navy's special forces element. Different parent service, different specialisms; sits alongside the SAS within United Kingdom Special Forces rather than under it.
What the Army publishes and what it doesn't
Roles, criteria and application routes are published. Operational detail is not, and anyone offering it to you on a forum is guessing, embellishing or lying. Treat the Army's own pages as the only source for current criteria.
Where to start if you're considering it
Attend a briefing event through the official route before you are fit enough. You learn what you are actually training for, and you will start the preparation months earlier than you otherwise would.
A note on accuracy
Entry criteria, standards and selection dates are set by the Army and change over time. Take anything current from the Army's own UK Special Forces Reserve pages; take everything below as one applicant's experience.
Get my 21 SAS reserve selection prep guide
What the stages are, what I was training to before I applied, and how I'd build the twelve weeks before a selection weekend. Written from my own application, free, no pitch.