USB drives mount question

[Ken Scott]

Hi all,
I have several USB flash drives and a USB harddrive.  Does anyone know
why When I plug these things in the reported dev sometimes differs. 
Sometimes it is /dev/sda or sda1 or sdb.  What is the  rule here?  I
also noticed that it works differently on different distros running on
the same hardware.  The same stick is assigned different devices in
different distros even though it is plugged into the slot. Anybody know
why?
Thanks in advance.
Ken