$SessionID is a number set up to be unique to a particular Mathematica session.
$SessionID should be different for different Mathematica sessions run either on the same computer or on different computers.
The value of $SessionID is based on $MachineID, as well as AbsoluteTime[ ] and operating system parameters such as the Mathematica process ID.
See The Mathematica Book: Section 2.6.3 and Section 2.13.5.
See also: $ModuleNumber, $ProcessID.