Proper pidfile usage

I am unclear on the need and the usage of a pid-file and I wanted to know开发者_C百科 what is the correct usage of a pidfile and what are the best practices surrounding it.

in linux, typically in /var/run there are pidfiles listed for all currently running processes and they contain, obviously, the process id. so when you use commands like ps aux, and they list the process id (PID) as a column, you may use them as a base of comparison