(Bearing in mind that the government has plans to change the school leaving age, so this system may have changed by the time your DC have to think about it)
After GCSEs, which all children take at 16, some children currently leave to take a job, others go into the sixth form to do A levels.
In the first year, they do AS levels, typically in 4 subjects, which are examined in several modules, either by exam or by coursework, leading to an overall grade from A (top) to E. Most students will go on to do further modules (A2) in the same subjects in the second year, though they may drop one, which will then stand as a qualification in its own right, as an AS level.
You apply to university normally in the autumn term of your second year, (though it is possible to apply as late as March)and offers will be made by universities on the basis of your results at AS, the grades your school predicts you to get, and the personal statement which all applicants have to submit. The offer will normally be for 3 A level grades & will vary according to the university and the course - a popular course at a top university will usually need 3 As, while a less popular course or university may ask for lower grades.
Once you've got all your offers, you can choose up to 2 - normally a higher offer as your firm choice & a lower one as 'insurance' in case you don't get the grades.
If you get the grades for your firm offer, or for your insurance offer you normally have to take the place, or withdraw completely that year & apply again.
If you don't get grades high enough for either (or if you don't have a second offer), you can go to 'clearing' - which is a system which offers all the spare places on all the courses at participating universities.
This is a very basic outline, as there are lots of other options for post-gcse, including the International Baccalaureate, vocational courses & NVQ courses, but I don't have a lot of experience of those.