WHAT IS THE DIFFERENCE BETWEEN 100 125 400 FILM?
WHAT TYPE OF WORK WOULD BEST SUIT 100,125,400 FILM.EG: SUNNEY DAYTIME / FIRE WORKS AT NIGHT?

The numbers represent how fast the film captures light. The higher the number the faster it catches the light, so it would be best used at night. The higher numbers also tend to have a grainier texture and don't do as well when they are blown up.

Those are the film's "ISO" ratings, used to determine the film's sensitivity to light. Your camera's meter usually picks up this info from the code on the film cassette. The highter the number, the more sensitive the film - this applies to B&W and Color.

A 'manual' method to use these numbers:
On a bright sunny day near midday, a film rated at '100' will require a manual camera setting of 1/100 th of a second (1/125 is close enough) at f.16. A '200' film will need, at f.16, 1/200 (1/250) seconds of exposure.

A doubling of the value indicates a doubling of the sensitivity. Therefore, a '400' speed film will allow the photographer to use a shutter speed twice as fast OR an f.stop smaller (f.22 instead of f.16 e.g.) than a 200 speed film.

With today's films, 'grain' is not much of a problem for films rated below 1600, so long as the processing is done properly.

As others have mentioned the ISO speed denotes grain, i.e. 100 would be finer than, say 400 but as has been mentioned already, modern film at 400 is OK. If you want the best of both worlds use 400 ISO like Ilford HP5 and buy a neutral density filter for bright days which will reduce light to film by say 4 stops. In poor light conditions take the filter off