For the purpose of aiding frequency management, the ITU has divided the world into three regions, named (unsurprisingly enough) Region 1, Region 2, and Region 3 (see map). Roughly speaking, Region 1 is Europe and Africa, Region 2 is North and South America, and Region 3 is Asia (excluding Asiatic Russia) and Oceania. The allocation of frequencies to the various services that a treaty member may make varies by region, mainly due to historical reasons but also due to different technological paths that were taken in different parts of the world. A full discussion of the details of the differences would be quite long; fortunately, hams don't need to know most of it.
The main situation in which differences in ITU region allocation has an impact on US hams is in 40 meters; the Region 2 allocation for the 40 meter amateur band is 7.000 to 7.300 MHz, while in Region 1 it's only 7.000 to 7.200 MHz and in Region 3 it's only 7.000 to 7.100 MHz. To make matters worse, many of the frequencies that are not allocated to hams in these regions are allocated to broadcasters. Since 40 meters is a worldwide band (much of the time), this creates a good deal of opportunity for interference. In addition, the subbands reserved for CW and data in each region are different, and until recently it was actually legally impossible for a US ham in Region 2 to have a phone QSO with a Region 1 ham on the same frequency (as the US phone subband doesn't start until 7.125 MHz, which was above the entire Region 1 40 meter amateur allocation). US hams wishing to work Australia, and until recently Europe, on 40 meters phone have to resort to "split operation", in which one station transmits on one frequency authorized to his use (but not to the use of the station he is calling), and the other station then responds on a different frequency that is within his authorization. The frequency pair to be used would either be agreed on by prior arrangement, or indicated in the initial call by specifying "split" and the frequency on which a response was expected (or sometimes by using "up" or "down" and the difference between the transmit and receive frequencies).
The astute reader will notice that just about all of the United States is in Region 2 (a tiny bit of US territory—Guam, American Samoa, and the Marianas—is in Region 3). However, §97.301 shows separate listings not only for Regions 2 and 3, but also for Region 1, even though no part of the United States is in Region 1. Why is this? Because of the "ship at sea" provision: FCC regulations apply to any person who wishes to operate as an amateur onboard a US-flagged ship while that ship is in international waters, no matter where in the world that might be. So if you're ever on a transatlantic cruise, and you've taken your ham radio along (and gotten permission from the ship's captain to operate, which actually isn't that hard to get), you'd better keep track of where the ship is; when it crosses over that line you may have to change your operating frequencies. (Of course, you have to keep track of where you area anyway, bceause once you enter the national waters of another country you become subject to that nation's rules.)
This post has been brought to you by pool question T1B02.