This is a portable gel system designed for use with speedlights / portable flashguns (note: it is not designed for the pop-up speed lights built into most SLRs, although I suppose you could adapt the supplied gels for this purpose if you really wanted to).
It comes supplied with a selection of different coloured gels, including CTO and Plus Green gels for balancing flash with tungsten and flourescent light respectively. The gels come supplied with a plastic holder which fits over the top of your flashgun; the holder has a storage compartment for all of the supplied gels and a clear plastic sleeve covering your flash which is where you install whichever gel you want to use.
The kit comes supplied with two peices of self-adhesive velcro (hook & loop) which you have to stick onto either side of your flashgun - these mate with corresponding bits of velcro on the holder itself, allowing it to be attached and removed quickly and easily. If you don't want to stick velcro onto your flashgun you can use an optional "Cinch Strap" (not supplied), but I would have thought a sturdy rubber band would do the job just as well. I went with the stick-on velcro option - it's much sturdier.
Once attached you'll find that the holder has velcro patches on the outside as well, allowing you to install other velcro-attached modifiers to your flashgun with the holder attached. This is a nice design feature.
I bought this for use on my Nikon SB-600 speedlight, and it fits very well and I'm very happy with it. The only slight niggle I have is that when the holder is installed you can't then flip out the built in diffuser on Nikon speedlights without removing and re-installing the holder. But that's a minor point - overall it's very well designed, and is of course aimed at a wide range of flashguns so it's not unreasonable that they don't allow for this particular feature of Nikon flashguns.
Note: the kit is designed for a wide range of flashguns, but there are some extra large ones that are not compatible - you'll find a compatibility table on the Lumiquest web site.