A PID is an acronym for process identification number on a Linux or Unix-like operating system. A PID is automatically assigned to each process when it is created. A process is nothing but running instance of a program and each process has a unique PID on a Unix-like system.
# you can use htop
apt-get install htop;
htop; # use cursor buttons to scroll up and down
boinc 1143 0.0 0.2 33064 9396 ? SNl 12:45 0:11 /usr/bin/boinc --check_all_logins --redirectio --dir /var/lib/boinc-client
boinc 2061 78.1 0.0 3388 3040 ? RNl 17:07 35:38 ../../projects/milkyway.cs.rpi.edu_milkyway/milkyway_1.40_i686-pc-linux-gnu
manpage of pidof: pidof.man.txt
now how to find out when the process was started/since when it was running: (actually i wanted to find out if boinc-client starts in the background before login… it does)
dr-xr-xr-x 9 boinc boinc 0 Jan 12 12:45 /proc/1143
so it was started Jan 12 12:45
hope this helps.