The most commonly used topo map covers 7.5 minutes, which equals 1/8th a degree of latitude and longitude. These maps are called the 7.5 Minute Series. Latitude lines are called parallels, which helps us remember that they are parallel to the equator.
The second number is the ground distance. For example, if your U.S. Geological Survey (USGS) map has a scale of 1:24,000, it means that one inch on the map is equal to 24,000 inches (2000 feet) in the real world.
The map (below) is Section 28 of the United States Soil Conservation Service, United States Department of Agriculture, (SCS, USDA) Soil Association map in Rapides Parish, Louisiana. For USGS topographic maps, 1:24,000 is the scale most often used.