Latitude And Longitude 

Assuming the Earth to be a perfect sphere, natural fixed points of reference are the North and South Poles. A natural fixed line of reference is the equator, the GreatCircle of points that are equidistant from the poles.
If we choose one arbitrary point on the equator, that defines a unique GreatCircle that goes through both poles and the given point. This Great Circle is called the Prime Meridian.
Now for each point on the Earth's surface we can define a pair of angles. The angle required to rotate the given point to the Prime Meridian is called the Longitude. Once there, the angle required to rotate the point to the equator (while keeping the Prime Meridian fixed) is called the Latitude.
Determining one's Latitude is comparatively simple using a sextant or octant. Determining one's Longitude was one of the grand challenges of the 18th century, and was referred to as the LongitudeProblem.