In the kit world, there are three types of F-packs.
1. The fruit flavour and sweetening type that is included with Mist wines. Liquid invert sugar with fruit flavours.
2. The Suss Reserve style is included with better quality whites like Gewurztraminers and Rieslings. To me at least, these seem to be grape juice/concentrate.
3. Sweetening packs for lower quality whites (Gewurztraminers and Rieslings) and blushes (White Zinfandel). Liquid invert sugar.
The above is generalized from what I have seen mainly in Vineco and Spagnols kits, but also other brands like Winexpert. Not every kit will have an F-pack. For example, in the Ken Ridge Classic line there are two blushes. The White Zinfandel has a sweetening pack, and the White Merlot doesn't. (Info up to date as of Aug 2007.)