What's the difference between the dialout and tty groups?

["Martin McCormick" <martin.m@suddenlink.net>]

	I added myself to both the dialout and tty groups on a
stretch installation in order to use the serial ports from the
user level.  It works fine.  Then I looked at a debian jessie
installation and found that I had only added myself to dialout
and it still works fine.

	What is group tty actually for?

This is more of an idle curiosity question than it is one of
something's broken and I need help.

	I am a retired systems engineer who setup unix systems
for coworkers to use so I know you can sometimes have too much
power at the user level and accidentally make it easier to make a
mess of things.

Thank you.
Martin McCormick