The type of slot the card fits in.
There are several different types of slots that video cards can fit in. Initially there was ISA, then PCI, then AGP 1x through 8x, and now PCI-E.
Currently almost any motherboard purchased within the last 2 years will support either AGP 8x or PCI-E. While most AGP specifications were backwards compatible with one another, PCI-E is completely different and will not work with an AGP card and vice versa.
On the chips pages, this refers to the best interface the chip was designed to support. Note that chips are sometimes bridged to other interfaces using a separate bridge chip. The interface listed on the chips pages ignores does not take the use of bridge chips into account.
A complete comparison of interfaces, their specifications, and how to tell the difference is coming soon.