What is session leader in `ps ax`

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http://stackoverflow.com – See the command bellow # ps ax | grep bitlbee 2271 ? Ss 0:00 /usr/bin/bitlbee -s mysql -c /root/config.conf 3930 ? S 0:00 /usr/bin/bitlbee -s mysql -c /root/config.conf See the STAT column. Here is the explanation S Interruptible sleep (waiting for an event to complete) s is a session leader What is session leader here? Also what is session here? How do I know which of these 2 process is active? (HowTos)