As far as I know it only reads MP3's off it. If you are looking into one of the entertainment versions it will also play DIVX Video, MPEG's, etc.

http://www.mobile.jvc.com/technical.jsp#feat_02

USB Memory
JVC head units can now be linked to PCs in even more ways. Save your favorite songs to a USB memory, plug it in to the JVC head unit, then listen while you drive. You can also enjoy songs from a friend's USB memory.

Note: "The USB port on all JVC models supports USB memory up to 4 Giga bytes. It should not be used to directly drive any device which derives its power from the USB port i.e. MP3 players such as Apple iPod."