Alrighty, here it is:
"AirPort" is Apple's trade name for 802.11b, which is an 11 Mb/s protocol over 2.4 GHz. They had one PCMCIA-based card that was available, and two base stations, the original "Graphite", and the slightly newer "Snow", that are both "UFO-shaped". Both models have a dial-up modem, the Graphite model has a single 10 Mb/s Ethernet port, the Snow model adds a second 10 Mb/s Ethernet port, but is otherwise identical, functionally.
"AirPort Extreme" is Apple's trade name for anything faster than that. It has been used in computers to denote an 802.11b/g card, an 802.11a/b/g card (the Intel Macs before 802.11n,) and an 802.11a/b/g/n card. The b/g card is a 'standard' card that Apple sold as an add-in for some computers. The a/b/g card and the a/b/g/n card have never been available for purchase alone. For 802.11b and g, these cards operate at 2.4 GHz. For 802.11a-compatible cards in -a mode, it operates at 5 GHz. For the 802.11n-compatible card, it can operate at either 2.4 GHz or 5 GHz. (Note that very few base stations support 802.11n's 5 GHz mode.)
In base stations, there was the original "AirPort Extreme", which was another "UFO-shaped" base station, this one supporting 802.11b and g. This model has a USB port for printers (no disks,) and was available with a modem and an external antenna port. There was also a Power Over Ethernet model available.
Then there is the current "AirPort Extreme with 802.11n" base station, which has two revisions. One has 10/100 Mb/s Ethernet (Apple calls this the "Fast Ethernet" model,) one has 10/100/1000 Mb/s Ethernet (Apple calls this the "Gigabit Ethernet" model.) The two n models support 802.11b/g/n in 2.4 GHz model, and 802.11a/n in 5 GHz mode. You cannot have it in both 2.4 GHz and 5 GHz mode simultaneously, you have to pick which frequency you want it to operate in, and that determines if you get -a or -b/g compatibility. Both revisions support USB printers and disks, but they lack a dial-up modem, or antenna port.
"AirPort Express" refers only to the small 802.11b/g, 2.4 GHz base station. This is the only model to support "AirTunes", and it also supports a USB printer (but not a USB disk.) This name does not refer to any cards, nor to the wireless technology itself, the way "AirPort" and "AirPort Extreme" are used. This term is used SOLELY for the one base station.