1. Verbal Reasoning
Basically a test of your verbal IQ
2. Numerical Reasoning
Basically a test of your numerical IQ.
3. Situational Judgement
This is an odd one. You get a series of questions about what you would do in various public-service type scenarios. Stuff like: "It's 6 on a Friday, your boss asks you to have a 5,000 word draft on her desk on Monday. Do you a) spend all weekend doing it; b) refuse to do it; c) look for an extension; d) look for help from other colleagues". These are scaled on a grade of more to less appropriate. It is very hard to know what the right answer is for these questions. I know people who score worse on these the more public service experience they have. To my knowledge these questions have never been back-tested against actual performance in public service
So to sum up: it's very hard to study for 1 or 2. A chat with someone who's done a lot of tests will give you a feel for what will come up in 3.