I have a few more points to add on the subject of time signatures.
My first point may seem a little picky, but it is true that a time signature is not a fraction. It is notated in common language quite frequently as a fraction, but in musical notation, you will notice, there is no line between the two numbers (see mblase's wu directly above). I would prefer if time signatures, in common language, would be notated with a dash instead of a slash (4-4 instead of 4/4), but most people would know what I meant by 4/4, more than they would by 4-4. And anyway, this is a pretty minor point.
Something that is a little more important to keep in consideration (especially in regards to the accented pulses noted by themusic and chrisjh), is that these "rules" are nothing more than conventions. There is nothing stopping a musician from playing 6/8 as if it were a simple time signature, or 3/4 as if it were compound. The same is true about the accents in the messure. These are the conventions followed by most composers and musicians of the "classical" fold, and can be used as the basis for musical knowledge. When expanded upon, these conventions loose all hints of meaning.
As a student of music, I am well aware that the above paragraph will be looked down upon by many musicians. I have gotten into many a fight with many a teacher on this very subject. As two examples of 6/8 being played as a simple signature (thus proving that any signature can be played contrary to the popular convention), I point to two songs, one easier to find than the other. David Byrne's Crash (off of his DavidenryB album) has an outro section that does this. On Live and In Living Colour, a live album recorded by Living Colour after the release of Vivid, as it was broadcast over WNEW from New York City, has an extended drum solo just before Money Talks. Will Calhoun does play 6/8 as a simple time signature during one part of that solo (for those of you who have this amazing performance, it's the part where he's playing along with the synth loop).