VS is the newest low-end series. I think they're mainly sold in developing countries.
The VX series is discontinued AFAIK.
CX is their general entry-level series. GS is an improved version of the same PSUs, with better components, longer warranty etc.; they're also targeted at gamers, with LEDs and such.
They're made by CWT.
TX is their mainstream series. The current versions (TX V2) are manufactured by Seasonic (generally better quality than CWT). While CX and GS are a mix of 80+ standard and 80+ bronze, the TX V2 PSUs are all 80+ bronze rated.
HX is the "worse" of their two enthusiast series. They're all modular, and rated for high efficiency (typically 80+ silver or 80+ gold). Some are made by Seasonic, others by CWT.
AX is...