Rab is right,
Review your kinetic molecular theory, there are four principles in the theory.
If my memory serves me right, they are
1.) Collisions are completely elastic
2.) The volume of each molecule of ideal gas is negligible
3.) There are no intermolecular forces present
4.) Random motion
So if you look at these carefully, you can have two types of deviations: those due to pressure and those due to temperature.
Low temperature causes the average velocity of the molecules to be less than ideal. In this situation you could imagine that intermolecular forces start to become more significant, making the volume less than would be expected.
High pressure, is not as easy. To date, I've yet to come across a question that asked me to differentiate between high and extremely high pressure. So, its probably practical that you understand the extremely high pressure situation.
In really really high pressure situations (like the MCAT) the molecules become really close, almost quite literally they're in each others' 'personal bubble'. So compared to when they were farther apart, the distance between them under these conditions is negligible compared to their own size. Its like sitting in the middle seat of an airplane, and seeing a 300 pounder walking down the isle. In your head your praying that they do not sit next to you. Because if they do, all you're going to be thinking about is how fat they are and that the distance between you and him/er is drastically smaller than the size of you two together.
So in this case, the molecules are taking up more space than predicted, because you now have to take into account the size of the molecules (or how fat the person is, but if they hadn't sat next to you, you wouldn't care how fat they were) when before you could assume their sizes were negligible.
Crude example, but as I was writing this I thought it would be funny.
