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Latitude
The spherical earth spins around an axis. The North and South Poles make up the ends of this axis. Latitude lines run horizontally on a map or globe, perpendicular to this axis. The equator line -- the central latitude line with a value of 0 degrees -- runs around the exact middle of the earth. Latitude values represent the distance between the equator and the respective poles. Positive latitude values ascend north while negative values descend south. Ninety degrees separate a pole from the equator.
Longitude
Longitude lines -- also called meridians -- run vertical on a map or globe, parallel to the earth's axis and perpendicular to the equator. These meridians pass from one pole through the other, essentially creating a great circle. Different cultures and societies throughout history used a different center longitude line, but as international dealings became more prevalent, international agreement held the center longitude at Greenwich, England. This line, called the prime meridian, marks the 0 degree point of longitude. Positive longitude values run east from the prime meridian while negative values run west, each reaching 180 degrees.
Measurements and Precision
Each latitude and longitude line represents a different degree, but for navigational purposes, latitude and longitude break up further into measurements of minutes and seconds. Each degree splits into 60 minutes and each minute splits into 60 seconds. Seconds divide up further into tenths, hundredths and thousandths when necessary. Approximately 69 miles or 111 kilometers separate latitude and longitude lines, but the distance between longitude lines gets shorter as they approach the poles. At the 45 degree mark, 49 miles separate longitude lines.
Application: Time
Aside from its navigational purpose, the use of latitude and longitude lines applies to how we measure time. When a line of longitude directly faces the sun during the course of the earth's rotation, every location along that meridian reaches 12 noon. The earth rotates by 15 degrees per hour, so after 24 hours pass, that same meridian faces the sun again. The abbreviation "a.m." refers to "ante meridian," meaning time that passed before the sun hits your meridian. The abbreviation "p.m." refers to "post meridian," meaning any time that falls after the sun passes over your meridian for the day.