Loosely speaking longitude and latitude are just related to the two angles in the spherical co-ordinate system. However, I was thinking about what angles are exactly measured by longitude and latitude. The easiest to understand link I could find was this (just the first half is relevant to longitude/latitude):
Nosco - Earth Coordinates
Essentially the xy-plane is the equator and z-axis is line through the poles. However, you would still have to choose a direction for x-axis (for longitude). It seems to be agreed upon based on a convention (described in the link).
Something that was bothering me a bit was the map (in the third/wikihow link posted by blastwave). How can one have lines of constant longitude and latitude as vertical and horizontal lines and still end up as a rectangular map? That's because if you change the latitude the rings/circles will becomes shorter and shorter. That is, if you were at equator you would have circle with the largest circumference. Increasing the latitude should make this smaller and smaller (hope it makes some sense).
So looking upon a bit, as one might suspect, the map isn't proportional in terms of distance/area. Something I didn't know about before.
p.s.
For those familiar with spherical co-ordinates one difference seems to be that latitude is measure as angle from xy-plane rather than as angle from z-axis.
