1. Galactic Latitude
This diagram illustrates how degrees of latitude are measured in the galactic coordinate system. The galactic plane is like the Earth's Equator, and like the Equator, it is at 0^o latitude. The Earth is on the galactic plane, so we are at 0^o latitude.
(Illustration: CXC)

Field Guide: Measuring Latitude

2. Galactic Longitude
Instead of going from 0^o to 180^o east and 180^o west, galactic coordinates simply go from 0o to 360^o. There is no east/west or plus/minus in galactic longitude coordinates. 360^o is the same as 2 x 180^o, or 180^o is half of 360^o.
(Illustration: CXC)

Field Guide: Degrees Longitude