VS 550w is a value series means its just for normal power supply.Different series denote different capability and the main thing is which series PSU give you how much ampire on 12 volt rail to supply enough power to your graphics.Also Bronze, Gold certification is measurement for good PSU
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 their top line. All modular, all rated for excellent efficiency (80+ platinum I think). They have various premium features, like the best fans and fanless operation at low loads. The AXi series (eg. the AX760i) add DSP for extremely good voltage regulation and ripple/noise suppression, and Corsair Link software control. I believe they're all made by Seasonic.