Each sector is identified by a small integer, such as 7. The simulation of each separate sector is handled by a simulation process called an ihost; ThereInc staff often refer to (for example) ihost7 as being the simulation server responsible for sector 7.
Sector 1 is where you are if you aren't in any other sector. The North and South Poles are in sector 1, as is most of the open ocean of PlanetThereia. The single ihost handling sector 1 uses different algorithms to the other sectors; however, a single machine is involved and building in sector 1 is officially A Bad Idea [source].
The International Space Station is in sector 99. As you fall to the ground, you pass through sector 10 and finally land in sector 50 (V2.06).
You can tell which sector you are in by enabling the hostHud in your ThereClient. Unless you are very near a boundary between sectors, or have just crossed a boundary, the SSOT value in that display is the sector you are currently in.
You can visualise the pattern of sector boundaries at ground level very clearly by enabling the trendSec option in your ThereClient, sometimes called QuintanaVision. This causes the terrain of each sector to be painted a different colour, rather than the "natural" colours for the area.
Observing the trendSec display, it is clear that sectors are usually rectangular at ground level. There are exceptions, though, where one smaller rectangular sector appears to be cut out of another larger rectangular sector.
[OpenQuestion: can sectors be completely arbitrary in shape, or is the irregularity we see caused by smaller sectors being in some sense nested inside larger ones?]
Sector boundaries are not fixed in stone forever, but can move from time to time. For example, the boundaries of sectors 7 and 11 (the Oasis and Sirocco areas) were moved for V2 of There. ThereInc have tools for moving sectors around dynamically. [V1] There is also a sophisticated system for adjusting them automatically in response to load, but that's turned off right now.