If you're doing GCSE, which you have indicated you are, foundation and higher are both prespective qualifications, just foundation is seen as less "valuable", supposedly, than higher as the highest you can acheive is a C (5 I think with the new grading system) whereas with Higher you can acheive a (9 etc.) plus Higher, offers more challenging questions as opposed to Foundation. And no you don't take them at the same time.