Hi outofhere:
I'm just a Mum, but as far as I know maths, literacy (reading/ writing) and science are all tested for. I'm expecting my DD2 to do all 3 exams this year, she's also in Y2. I know some testing has started and my DD is blithely unaware of what it is all about. It's just something interesting she did at school that day (or not that interesting as the case may be).
A good way to gauge how your DC is doing with all this is to use the BBC Bitesize practice games in maths, literacy and science: www.bbc.co.uk/bitesize/ks1/
There's been a lot of discussion on MN of late and I think I'm being fair to summarise by saying that this isn't a test you should worry about or try to prepare for. It's probably better to view this test as an independent check that the school is doing its job.
In my case DD1 didn't attain the national expectation 2b, but she's gone on (with a lot of help at home) to do very well. As a parent, I think the best approach is to make no fuss about this at all - maybe play a few video games, but to be aware that you should be informed at some point how your DC did. At this point you can use the results as a way of gauging how your own DC (with their own issues) is fairing at school so far. In our case it was a wake-up call that whatever was happening at school, it wasn't really working for our DD. We then started to do more at home, which has made a big difference.
The NC Scores are explained on Mumsnet Learning pages here: www.mumsnet.com/learning/assessment/national-curriculum-levels & follow links to other pages explaining what the levels mean and how a child is expected to progress through the levels.
Basically by the end of Y2 the expectation is that children should be working to Level 2b or higher. The prediction then will be that by Y6 they should improve 2 full National Curriculum Levels - attaining a Lebel 4b at age 10/11 end Y6.
If a Y2 child scores a 2c and goes on to attain a 5c - the school will get AVC (added value) for helping children to attain beyond expectation.
If on the other hand a Y2 child scores 3b and then only achieves 4a in Y6 - this would be below the 6 expected sub-levels (or 2 full NC Levels) and therefore the school would lose AVC points.
HTH