The supply situation of the ISS is quite interesting.
Soyuz[1], the Russian spacecraft, will continue bringing and returning people to and from the ISS (three each – seven is a typical Space Shuttle crew). Two Soyuz will continue to be permanently docked to the ISS (they will, of course, be regularly switched out whenever a new Soyuz flies to the ISS) and serve as lifeboats for the ISS’s crew of six. They are actually the limiting factor for the crew size – an evacuation of everyone on the ISS has to be possible at all times.
Soyuz will be the only spacecraft that can return something to Earth after the Shuttle retires. Since it’s designed to fly people and not cargo it’s certainly easy to imagine that Soyuz is far from ideal for this job – forget bringing back any big experiments.
Progress[2] is the unmanned cargo version of Soyuz. It will continue bringing cargo to the ISS. (Two to three tons per flight. The shuttle could bring about 25 tons to low earth orbit.) It’s, just like Soyuz, a very capable and reliable work horse. (It was the only spacecraft bringing cargo to the ISS when the Shuttles were grounded between 2003 and 2005.)
There are two other and newer cargo spacecraft: The European Automated Transfer Vehicle (ATV[3]) and the Japanese H-II Transfer Vehicle (HTV[4]). The ATV automatically (like its name indicates) docks to the Russian part of the ISS and can bring up more than seven tons of cargo. It can also boost the station into a higher orbit (as is regularly necessary). The second ATV Johannes Kepler only recently successfully finished its mission, was filled with trash, undocked and burned up in the atmosphere.
The HTV can bring about six tons of cargo to the ISS. It has a pressurized and an unpressurized cargo section (meaning anything that’s supposed to be attached somewhere outside the ISS doesn’t have to go through and fit through the airlock, it can be brought to the ISS in the unpressurized cargo section). It has no complicated automated approach system like the ATV, it is simply grappled by the Canadian “robot” arm (well, it’s more a crane than a robot, really[5]) and attached to the American part of the ISS. Since it’s docking to the American part of the station, the docking hatch is large enough to accommodate International Standard Payload Racks[6] which are used everywhere in the non-Russian part of the ISS. The ATV can’t bring those racks to the ISS since it docks to the Russian part of the station (and their hatches are too small). The HTV also only recently successfully finished its second mission and burned up in the atmosphere.
That’s the current situation, I will leave it to someone else to explain what the future might hold (SpaceX’s Dragon spacecraft, for example). Bringing cargo back from the ISS is the only real problem, there will only be Soyuz after the Shuttle retires.
The ISS can currently also not be extended, well, at least not the non-Russion part. Bringing up all the modules was a job the Shuttle was perfectly suited for. That’s no problem since the ISS is done. They actually managed to pretty much complete it, even looking at the original plans. Heck, even the Cupola[7] made it.
[1] http://en.wikipedia.org/wiki/Soyuz_(spacecraft)
[2] http://en.wikipedia.org/wiki/Progress_spacecraft
[3] http://en.wikipedia.org/wiki/Automated_Transfer_Vehicle
[4] http://en.wikipedia.org/wiki/H-II_Transfer_Vehicle
[5] http://en.wikipedia.org/wiki/Canadarm2
[6] http://en.wikipedia.org/wiki/International_Standard_Payload_...
[7] http://en.wikipedia.org/wiki/Cupola_(ISS_module)