If you don't play an instrument, or only play by ear...OR you're new to this whole songwriting thing...here are a few terms you may hear and what they mean.  These are just a few musical terms, for songwriting terms, see below.

Bar - A piece of music or a song is broken down into sections called bars.  If you've ever looked at sheet music, the bars are defined by dark vertical lines (hence the term "bars") and between each bar is a set number of notes.  You've probably heard the term "4 beats to the bar".  Bars help to define how long chords or notes are played (i.e. two bars of G).

Time Signature - This defines how many beats there are to a bar, and the value of each one.  3/4 time is recognizable as a waltz-type feel (ONE, two, three, ONE, two, three).  The most common time is 4/4...in fact, it's even CALLED Common Time!

Lead Sheet - This is a very basic kind of sheet music.  Most lead sheets have only the bars written out and the chords within the bars and some simple notes.  This is often used when you go into the record, and is used by each musician to follow along.  Lead sheets don't necessarily have lyrics written on them, although some do.  

Sharp/Flat - You've probably heard the reference to singing "flat"...this means that you're singing just a little lower than the note should be.  Singing sharp means it's a little too high.  I won't go into the theory of sharps and flats, but it helps to be able to identify which is which!

Octave - a standard scale has 7 notes (not including all of the sharps and flats) before it reaches the same note higher up...think of Doe Re Mi Fa So La Ti Doe...both "Doe" 's are the same note, but the second one is an OCTAVE higher than the first.

Vocal - the voice!  There are "lead" vocals which is basically the singer singing the melody, and there are background vocals, singing harmony, etc.

Lead or instrumental -  Sometimes in a song there is a section where an instrument plays for a few bars before the vocal starts again...this is called a lead or an instrumental section.

Fills - These are little bits of music that are often played by a lead instrument to "fill" in a space on a recording or performance.  When you ask a guitar player to play some fills, he/she will usually weave some notes in and around the melody or vocal, just to add a little flavour.