Distance per degree
Latitude and longitude refer to the coordinates of a location on Earth.
Latitude is how many degrees north of the equator it is.
Longitude is how many degrees east of Greenwich it is. (Greenwich is a place in London.)
Order when written
When two numbers are given, not indicating which is latitude and which is longitude - the official order
is that the first is latitude. However, that is not always the case. Note that sometimes it is indicated with a letter after the number indicating North, East, South, or West. In those cases, South and West may be given as positive numbers.
Source of confusion
Diagrams with horizontal lines parallel to the equator are drawn associated with latitude, and vertical lines from pole to pole are drawn associated with longitude. This is counter intuitive since we are used to seeing a line represent some component as it changes, while here the lines represent locations where the component stays the same.
Why are lines drawn as they are? Because they are drawn so on globes. And why draw them there? So that a person can look at a point at the globe and tell what are its coordinates. For example a person looking at London on a globe could see it's around 50N 0E (50 North, 0 East) by looking at those lines.
Those lines have names. The lines indicating a specific latitude are called parallels (because they are parallel to one another), and the ones indicating a specific longitude are called meridians, and they are not parallel to one another, they connect the north and south polls. The "Prime Meridian" is the one that passes through Greenwich and indicates a longitude of 0.
Distance per one degree
Note that while the distance per one degree of latitude is more or less constant (around 111 kilometers), it is not so for longitude. To see this, image standing at a point just one meter south of the North Pole, which simply means standing a meter away from the pole. Now start walking east until you get back to your starting point. You will have traveled 360 degrees of longitude, while walking only 2π (a little over 6) meters in a circle around the pole.
The distance per one degree of longitude at a latitude of θ⁰ is COS(θ⁰) times the distance for latitude mentioned above (~111km).
A degree of long
itude changes in how LONG it is (depending on its latitude) while a degree of latitude stays the same length.