Overwatch

Overwatch is the general name of the Combine's military forces on Earth. It is used as a suffix designation for the forces in the various sectors such as Sector 17 Overwatch for City 17 and the surrounding countryside as well as being the specific name of the automated Command and Control System on Earth, and possibly similar systems elsewhere, which is most commonly encountered in the form of the Combine Dispatcher / PA. Which the primary military is the humanoid forces with support by Synth units

Known Sectors and branches of Overwatch
The Overwatch having complete control of Earth have placed all of the surviving humans in Sectors/Cities where they are re-educated to serve the Combine. The Combine also have Sectors of the humanoid military such as Sector 17 Overwatch which is the only Combine military sector Gordon encounters.

Sector 17 Overwatch
Sector 17 Overwatch is the official designation of the division of Overwatch Soldiers stationed in City 17 and the surrounding environs, including The Coast and Nova Prospekt. The designation was revealed by Dr. Breen during Gordon Freeman's venture into Nova Prospekt's B2 sector in a speech congratulating them on the capture of Dr. Eli Vance. Also of note, he expresses severe "disappointment" in the fact that they had repeatedly failed to capture Freeman, pointing out that they are highly trained soldiers, while he is only a scientist.

This also suggests that the Combine have divided Earth into sectors. Since City 17 is located in Sector 17 than it also suggests that these sectors are nothing more than Combine-controlled cities and their surroundings. According to this logic there should be as many sectors as there are Combine-controlled cities.

No one knows the name of the other Combine forces however it's likely that those of them which are stationed in other Combine-controlled cities carry similar designations with the name of their respective city/sector. Episode Three should give a clue to other Combine forces with different names and if they have different or new units other than the regular Overwatch Synth and soldiers. Sector 17 Overwatch also contain branches for handling a particular situation.

Airwatch
The aerial forces of the Overwatch are tasked with attacking and destroy resistance installations or members. These forces while not having a large selection it has deadly vehicle and Synth units which includes the Hunter-Chopper and the Gunship.