A tactical light is a flashlight used in conjunction with a firearm to aid low light target identification, allowing the marksman to simultaneously aim and illuminate the target. Tactical lights can be handheld or mounted to the weapon with the light beam parallel to the bore. Tactical lights also serve a role as a method of non-lethal force, used to temporarily blind and disorient targets.
What exactly is a tactical flashlight?... And besides cost... how is it any different than any other flashlight?
If you don't have a tactical light, all the other urban warriors will laugh at you and make fun of you. You will be branded as an incompetent by the tactical establishment. Black-clad ninja will leap from the shadows at the first opportunity and you will be powerless in the murky darkness. Your family and friends will be forced to live with the shame of your poorly lighted demise
Humor aside, that is a good article on the tactical flashlight.
As far as differences from other lights, Tactical Lights are usually relatively bright LED with forward clicky tail-caps and several functions, low, high, strobe, etc. They are also small enough to carry in ones pocket, or mount on a weapon. And yes of course a regular flashlight could be used for self defense, although they weren't designed with that in mind.