♫ On the 4th day...

...of Christmas, my Zombie gave to me: ♪

4/4 time
3 Pips and Gladys
2 Live Crew
and Tom Waits in a treeeeee!

4/4, or common, time means that there are 4 beats to a measure and the quarter note is the beat.

It's easiest to reinterpret the lower number in a time signature as a fraction: 1/4 => quarter => quarter note.

A quarter note.  Observe the filled in head and stem.  The stem may go up or down.

I promise that I'll learn some sort of Photoshop or drawing program someday.  For now, though, you get my low-tech drawings.  Zombies can only do so much at one time.  Braaaaaaaaainsss...

 When placed into a measure (which is really just a way to organize the sound into a readable, easily comprehensible format) you get something like this:

More low-tech music notations.

You will remember that "beat" is the pulse that you would tap your foot to.  Rhythm is the interesting stuff that happens over the beat.  Note (hehe) that there are 4 quarter notes inside this measure.  Four beats of music, with the quarter as the beat.  Tadaaa! 4/4 time!

There are other time signatures, but the formula to figure them out is always the same.  The top number (which can be anything, really) tells you how many beats are supposed to be in a single measure.  The bottom number can only be one of a few numbers (because there are is a set amount of kinds of notes).  So, with this in mind, we know that the time signature of 7/16 would result in 7 beats within a measure, with sixteenth notes as the beat.  We also know that music in this kind of time signature would be annoying, at best, and would be impossible to march to.