The /proc filesystem gives information about all processes running. It is read-only.
With a 2.4 filesystem, /proc also contains some special files to obtain some info about the current kernel configuration. Some files also allow root to change some settings on the fly in the kernel; those are of course writable by root. With a 2.6 kernel, /proc has been cleaned up and should only contain processes-related stuff; kernel-related stuff should be in /sys.