The name of the generation of cards this card comes from. Typically, video card manufacturers release cards as a group ranging from low-end to high-end models. The 'Related Cards' section on the card pages lists cards which share the same 'Series'.
Video cards of the same series typically share a common feature set and are derived from the same technology. Video card manufacturers develop a series of cards usually starting at the high end to generate as much buzz as possible. From there, they take their high end design and come up with variations on it to produce a bunch of different cards they can sell at every pricepoint.
Manfacturers accomplish this by varying the core and memory speeds, and the number of pipelines in the card. For example, a low end card might have 2 pipelines clocked at 300Mhz, whereas a high-end card in the same generation would have 24 pipelines and be clocked at 600Mhz with much faster memory to match.
The important thing to note, is that despite the differences in speed and pipeline count, the actual features supported by the cards in the same series doesn't really change much (with the exception of some high-end features being disabled in low-end cards which lack the power to use them, like anti-aliasing).