A super-selective school is one which allocates its places in order of pass mark and has its own entrance exam. So, if a school has 120 places it will take the 120 children who scored the highest marks in the entrance exam.
In selective areas, children sit an 11 plus provided by the local education authority. For example, in Bexley, all year 6 children sit the 11+. Each year the pass mark varies and ALL children who achieve the pass mark are deemed selective. However, once the sibling/special needs etc criteria have been applied, the main deciding factor is distance to the school. So, for example, at a grammar school in Bexley, a school with say 160 selective places will take the 160 children living closest to the school who have passed the 11+ regardless of their scores.
Some areas, like Kent, have lea-run 11+ tests operating on similar lines, but also have super-selective grammars (like Judd and Skinners?) which run their own entrance exam and select those children with the highest scores.