Almost all the dance music I play and listen to has 4 beats in each bar (if you count along to it, it's natural to count 'one, two, three, four, one, two, three, four,....').

It's also normally built up in chunks of 8 bars, so if you count the bars, after 8 bars have finished, something normally changes about the music, like a new sound might be bought in.  Bigger changes tend to happen every 16 bars, and even bigger ones every 32 bars etc.  This is all in very general terms, different tracks are built up in different ways, but they normally follow this general idea – which makes them nice to dance and listen to as there's an air of expectation to the music.  Anyways, knowing about this is important, as you have to try to keep this sort of structure to the music when you're mixing from one track to another – so as well as matching beats, you will normally be matching bars – eek!