Difference between Java memory limit and server memory

[lovebags]

Is anybody able to explain the difference between the between 'Java memory limit' eg: 512Mb (which I have set in the Subsonic taskbar in Windows) and the memory as listed at the end of 'Server' in the 'About' page eg: jetty-6.1.x, java 1.6.0_18, Windows Server 2008 R2 (94.5 MB / 144.8 MB)

I'm guessing that 94.5 Mb is memory used by the application but what does the 144.8 Mb stand for and how does it relate to 512 Mb?

Also what actually causes memory to be used by the application? I always thought it was when a song is played as I can see it go up by about the same Mb that the song is. Are there other things can cause memory to be used up?

I've had various issues over the years when the server stops responding during high use (with up to 50 users at once) and my feeling is this is due to running out of application internal memory rather than available server memory or CPU.