The National Curriculum of England is a system put in place by the government to maintain an acceptable standard of (and access to) education for all children aged 16 or under. Here is a quote from the British government's website:

"The National Curriculum sets out a clear, full and statutory entitlement to learning for all pupils. It determines the content of what will be taught, and sets attainment targets for learning. It also determines how performance will be assessed and reported."

The national curriculum features a series of achievement levels from level 1 to level 10, which are what is usually concentrated on in primary and early secondary school. A 5-year-old just starting in primary school reception class is assumed to be at level 1 and have pretty much no skill in any subject. Students are expect to go up a level in each subject every two years. Therefore, an 11-year-old (in last year of primary school, year 6) is expected to get a level 4 in a given subject.
The national curriculum requires children to take SATs (national Standard Attainment Tests, little if any resemblance to the North American version) at the ages of 7, 11, 14, and 16 (though the SATs taken at 16 are known as GCSEs). The SATs taken at the ages of 7, 11, and 14 are in three subjects, English, Mathematics and Science.

Interestingly enough, even though SATs normally only test a student in three subjects, the national curriculum defines targets, albeit less exact targets, in eight other subjects:

Part of the national curriculum are semiregular school inspections, with inspectors sitting in on lessons, handing out questionnaires to find how satisfied pupils are, and asking teachers what they think of their school. Surprisingly, neither the teachers nor the students actually lie to make the school look better to the inspectors, probably because it's OK as long as there aren't huge fights breaking out in the classrooms.
These inspections could take place as frequently as every couple of months to only every five or six years for especially Good Schools.

The national curriculum also tries to include help for those with learning difficulties via a Special Educational Needs system. Sometimes it helps, sometimes it doesn't.

Interestingly enough, the national curriculum itself is a perfectly adequate system of education, but it tends to be the implementation of it which is usually poor. This tends to manifest itself in poor SATs results, allegations of discrimination and exams being too easy/too hard, violent pupils, and other such things.