Unix library files are akin to Windows .dll files. Files here are stored in memory, not on the drive. A user or admin can get information on programs running through the proc files. Admins like to delete stuff here when running low on diskspace. On some systems, this directory is completely erased during bootup.
Logs keep track of connections to the system, daemon activity, file transfers, mail, and more.
Programs like 'ssh' and 'pgp' are typically installed here by default.