The hierarchy is (from lowest to highest): C, X, S, G, V
The C range are cut down models with worse filters applied, less connectivity and generally poorer build quality. However, they're cheap.
The X range doesn't have the much touted "full HD" 1920x1080 resolution, but otherwise are highly capable sets. I'd definitely go for one of these over a C series model if that's the kind of area you're looking at.
The S range is 1080p, but not using the latest panel technology (for plasma) or 100Hz technologies (for LCD models).
The G range is basically where Panasonics current proper range starts. It uses their latest neoPDP plasma panel, has a freesat tuner, etc. (The G15 adds Viera Cast, explained below, and some slight design differences over the G10, they are otherwise identical).
The V range has (much) nicer design, an out-of-the-box accurate THX picture mode (on plasma models) and also a bunch of networking features (when wired into a network you can access YouTube, Picasa web albums and others. These features are known as Viera Cast).