Internals of /proc in linux

/proc is a temporary filesystem which linux initializes, in order to keep track of resources and states of each process, as well as system wide resources. Every command line tool like ps,top,vmstat,free derives its results from parsing /proc directory.

$ free -m 
           total     used     free   shared  buff/cache   available
Mem:        3771     2921      276      138      573      450
Swap:       6355      667     5688

$ vmstat 
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
0  0 683248 323280  75244 514304    2    3    15    42   62   49 22  2 76  1  0

1. /proc/pid represents per process resources statistics. It may give you list of file descriptors pointing to files or sockets, opened by this process.
2. /proc/net is a virtual place for everything related to network in your running operating system.
3. /proc/sys is place where you can set or view typical limitations of your systems resources. Like, maximum number of pids, size of pages etc.
4. /proc/irq is a place for interrupt requests. You can taskset, or map irq handling to particualr cpus.
5. /proc/sysvipc relates to System V IPC calls for messages (msg),semaphores (sem), and shared memory (shm).

In all, /proc is a very interesting directory of linux. And with little investigation, you can run interesting experiments.