excerpt from Linux Filesystem Hierarchy
/procis very special in that it is also a virtual filesystem. It's sometimes referred to as a process information pseudo-file system. It doesn't contain 'real' files but run-time system information (e.g. system memory, devices mounted, hardware configuration, etc). For this reason it can be regarded as a control and information center for the kernel. In fact, quite a lot of system utilities are simply calls to files in this directory.
It provids a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory.