start up disc full but not sure where the space has gone

I am using a Mac Book Pro 15 inch early 2011
running Mac OS X lion 10.7.3
I have 10 GB memory
and 500 GB of storage.
I am trying to use FCP X but when I use it it tells me that my start up disc is full. All the footage and projects for FCP are on an external drive which has 341 GB free
MY Mac HD is showing only 37 GB free out of 500 but I'm not sure what is using the space
I have attached a pic showing what is using up my storage but basically
Movie = 29GB
Audio 99GB
Photos 1
Apps - 13
Backups - o
and other 417

What is this "other?" and how do I clear it
I've done all the usual things like empty trash and delete old iTunes libraries etc - but it appears that my files of music/documents/photos and movies are not the issue? What else could it be?
Thank you

Karen

Please read through our Blog which explains lost hard drive space, especially "other". LINK

Grand Perspective is a good tool for finding where all your space has gone. The bigger the square, the bigger the file.
GrandPerspective

GrandPerspective just updated yesterday, incidentally!