Project Database (PDB)
PDB (Package Database?) files are used by the PS3 to specify tasks for the XMB to do. They are stored in /dev_hdd0/vsh/task/ in 4 byte hex directories (e.g. 00000001/, 001ABCD0/)
Each directory represents a task and each task is defined by a couple of .pdb files. I believe pdb means package database, since the only place where these files appear is in /dev_hdd0/vsh/task and they seem to represent a .pkg download.
There will always be a 00000001/ directory with no .pkg file in it. I suppose it's there to represent another task the PS3 has to do (maybe the 'check for push downloads' for PSN+ users?), and I suggest never to mess with it.
Every other directory seems to represent a .pkg file that is being downloaded or has been downloaded (but not installed) through PSN. In each directory you should find the .pkg, a PNG image to show (filename ICON_FILE), and two or three .pdb files (d0.pdb and d1.pdb and an empty f0.pdb, which may or may not be there).
The file starts with a 4 byte magic of 0x00000000
More will be updated soon...