Latitude vs Longitude
Latitude is the north-south coordinate, ranging from -90° (south pole) to +90° (north pole). Longitude is the east-west coordinate, ranging from -180° to +180°. The equator is 0° latitude; the prime meridian (Greenwich) is 0° longitude.
Latitude and longitude together name any point on Earth. They look similar but mean opposite things — one is north-south, the other east-west.
Latitude (φ): north-south
Latitude measures your distance north or south of the equator, expressed in degrees from -90° (south pole) to +90° (north pole). Lines of latitude (parallels) run east-west around the globe. The equator is 0°, the Tropic of Cancer is 23.4°N, the Arctic Circle is 66.5°N.
Longitude (λ): east-west
Longitude measures your distance east or west of the prime meridian (Greenwich, London), from -180° to +180°. Lines of longitude (meridians) run north-south from pole to pole. Each 15° of longitude corresponds roughly to one time zone.
The convention
When writing coordinates, latitude comes first: (40.7128, -74.0060) means 40.7128°N, 74.0060°W. Some systems (like KML) reverse the order — always check.