The pass/fail marks are incredibly strict but in terms of how difficult these marks are to attain depends on your branch choice. Your aptitude test will consist of around 20 computer-based tests designed to measure, amongst other things, your hand-eye coordination, mathematical ability, ability to interpret data, reaction speed and spatial awareness. Each of these tests has a different weighting for each branch choice. So, for example, if you're going for pilot then hand-eye coordination is a biggy aswell as maths and the ability to work under pressure at high speed. Sounds scary, but everyone finds it hard so don't worry!
I would recommend, whatever you're going for, that you practise quick mental arithmetic (brain trainer on the Nintendo DS is actually very good for this) and speed/distance/time calculations. There's a really good thread on this forum with a huge list of practise questions. DO THEM ALL.
I did my aptitude test in advance a couple of weeks ago so it's all still pretty fresh in my mind. Feel free to ask any questions!