What is the coordinate system used in an Aircraft?
Stations (STA), generally Fuselage Stations (FS), designate location along the length of the aircraft, increasing from nose to tail. Generally station 0 is somewhere in front of the airplane. One reason for this is that if the airplane grows longer, there still won't be any negative stations, and sections of the plane keep their station numbers. FS 100 can also be placed at an important location (looks like the firewall below).

Water lines (WL) designate location in the height of the aircraft, from ground up. You can probably guess by now that these are holdovers from the marine industry. Like the stations, water line 0 is generally a bit below the aircraft, possibly to have WL 100 at the center of the fuselage, or WL 0 could be at the bottom of the fuselage.

Butt lines (BL) designate location left/right on the aircraft, generally centered in the middle.

While the main system is based on the whole airplane, different parts (like the wings) have their own system of STA, WL, and BL.

The stations are generally in whatever unit is being used for the aircraft. In the US, this is inches.

See this link for an example and further explanation.

For a picture similar to the one below but with more detail, see here.