Momentary--Push for light, generally twist tight for "on".
Clicky (or forward clicky)--1/2 push light, release off, full push "on", click again off--
Generally, the Momentary and "forward" clicky are considered to be "tactical" clickies (used by police, military, and for self defense).
Reverse Clicky--Full press click "on", 1/2 press momentary off, full press--full "off".
Reverse Clicky is generally used with lights that have programmable output levels--i.e., click light "on". 1/2 click again and release (off-back "on") and you get another level. Full click, and light turns off...
There are lots of other mix and match combinations--and some folks believe forward clickies may be more reliable, and other issues about complexity of user interfaces (UI)...
In the end, much of this is personal preference and the use to which you will put your flashlight.
At times, clicky discussions approach religious fervor. Be warned...
-Bill
