Scout - a very good question to which I suspect the answer is actually 42!
The SATS are assessments done in English school at the ages of 6-7 (year 2) and 10-11 (year 6) in English and Maths. In year 2 they are totally teacher assessed and will be based on the work the child has done through the year, but the teacher may set some special pieces of work, done maybe with no talking between children, in order to inform her assessment.
In year 6, English and Maths are tested by external tests done under exam conditions, although the writing component of English is to be assessed by the teacher from this year. Abilities in science are teacher assessed
Pauses to wipe marmalade she has just dropped, off keyboard check you are with me so far!
In theory, they are used to check that each child is making the recommended amount of progress and, via looking at this, that the teachers/school are doing a good job.
League tables are published showing what % of children achieve which levels for each school.
Sats scores at year 6 will be used to provide a base level to decide on target levels of achievement for a child at secondary school and secondary teachers/schools are judged on how manywell children meat these targets.
In theory they provide a level of accountability for teacher adn schools.
In practice - a different story.
Some schools manage sats well (often the schools/teachers that were managing/would have managed well before sats came on the scene!)
Others manage to create loads of stress for the children and can spend most of year 6 focusing solely on sats subjects and "teaching to the test" to the detriment of history, art and pe. Even within the Sats subjects, it means that children don't always get the best sort of teaching/help imo!
For example: Imagine a girl of 11 who has suspected dyslexia and her reading (pure decoding of words) skills are 2 years behind where they should be. She may well have spend months in school since Christmas this year repeatedly practising sats style papers where she has to read passages and answer questions, some using inference etc. She can make inferences really well from information, her problem is actually deciphering the text but she is not getting any/much phonics instruction because they "need to practice for sats". I get very cross at this point as I can't see the point in making a child answer questions on repeated passages she can hardly read!! Similar things can happen with maths and it can just lead to children losing confidence!
In addition, if my anecdotal evidence is anything to go by, some teachers can cheat be very creative in the support the offer to children doing the sats!
Finally the whole thing is massively political and is used by whichever government is in power to show that:
a) teaching standards were crap until they took over
b) they have massively improved since they took over
There are some good points about sats and I think it is good to have some idea of the curriculum children should cover and levels that should (on average) be attained but in general, as they stand, I think they do more harm than good and should be abolished! They focus too much on the curriculum and not enough on the child!