Hi Hotdog,
I used to run psychometric tests for a company I worked for. It's just a means of measuring your personality, stengths, weaknesses, skills and abilities etc. The ones I used to run covered personality profile, career profile, spelling, speed & accuracy, mechanical reasoning, numerical reasoning, logical reasoning, spatial reasoning and verbal reasoning.
They're nothing to be afraid of - the tests they use depends on the type of job you are going for. You can't really prepare for them as such, but if mathematics is involved in the job, you could brush up on the basics of maths such as fractions, decimals etc in case you have a numerical test.
I don't really have an awful lot of faith in the personality/strengths/weaknesses profile because a lot of people will give the answers that are expected of them to make themselves sound good. Also, you can learn what type of answers are the best and what employers want to hear etc so the test results can be flawed.
You can't really cheat with the aptitude tests though, because you are either good at maths, mechanics, spelling etc, or you're not.
Hope it all goes well with you.
Terri