A Session is a collection of ProcessGroups. A newly created process joins the session of its creator. In normal operation the login shell creates a new session and all processes are members of that session. The login shell is the Session Leader.

Lets take a look at the SID's (Session ID's) of processes belonging to two separate ssh sessions on the same machine.

bss7@xxxxxx:~$ ps -jfu bss7
UID        PID  PPID  PGID   SID  C STIME TTY          TIME CMD
bss7      5595  5520  5595  5595  0 18:19 pts/70   00:00:00 -bash
bss7      6632  5595  6632  5595  0 18:20 pts/70   00:00:00 ping 192.168.1.1

bss7      6852  6851  6852  6852  0 18:20 pts/83   00:00:00 -bash
bss7      9954  6852  9954  6852  1 18:24 pts/83   00:00:00 vim
bss7      9993  6852  9993  6852  0 18:24 pts/83   00:00:00 ps -jfu bss7