MrXLA has given an excellent overview, presumably because he's within the trade, so to speak.
In order to complete their training and progress up from ST1 to ST3 and beyond, there is a series of exams and competancies which they have to complete. Demand tends to exceed supply of the number of positions available working under a consultant. At times there tends to be a bit of an 'old boys club', i.e., the junior who the consultant(s) favoured tends to get the job ahead of other candidates. However, this is because the consultant has usually taken them under their wing and they become a bit of a prodigy for the consultant. Also people who tend to specialise in say surgery have to move around during their ST years gaining experience in different areas of surgery, i.e., vascular, breast, colorectal, upper GI. To become a consultant surgeon they have to become a member of the FRCS (Fellow of the Royal College of Surgeons), likewise for other specialities.
Personally I dread August as some of the new FY1 come out of uni with an arrogance and attitude that defies belief. The adage don't get ill in August is so true.
Also the same applies to October when the new ST's comea round wanting to demonstrate their new found skills (or not as the case maybe).
Oh the joys og the NHS!