Latitude is a measurement of the distance North or South of the equator. Latitude has units of degrees, and is the angle required to form a triangle with one corner at the center of the earth, a second corner at the surface of the earth on the equator, and the third corner at the surface of the earth at the desired point North or South. A positive number is North, a negative number is South of the equator. There are two common formats, one with a leading plus/minus sign, and the other format with a "N" or "S" either before or after the number. In the plus/minus format, the plus sign is often omitted for North latitudes.
The continental US ranges in Latitude from about 26N (+26) (the tip of Texas and Florida) to about 49N (+49) (the US/Canadian border along Washington, Idaho, Montana, North Dakota, and Minnesota).
Longitude is a measurement around the earth, starting in Greenwich England/United Kingdom, near London. Longitude is positive as it travels East, and negative as it travels West. The same two formats are used with Longitude, plus/minus sign or "E" and "W" indicators. In the plus/minus format, the plus sign is often omitted.
The continental US ranges in Longitude from about 66W (-66) (the tip of Maine) to about 124W (-124) (the Pacific coast along Washington, Oregon, and California).
A Latitude/Longitude pair, often abbreviated lat/long, can describe any location on the surface of the earth. From a mathematical perspective, the lat/long pair does not imply the surface of the earth, but actually describes a vector, or a line, starting from the center of the earth and traveling outward. Where this vector, or line, crosses the surface of the earth is the point generally referred to by a lat/long pair.
In a lat/long pair, the Latitude is always first, followed by the Longitude. The "N", "S", "E", "W" indicators can come before or after the numbers, though a plus(seldom) or a minus sign always preceeds the numbers.
An interesting bit of trivia, the word "news" is supposed to have come from the four directions placed on a coordinate system in the heading of newspapers of old.
ex: 40.087659N, 104.980459W, or equivalently 40.087659, -104.980459
ex: N40 05 15, W104 58 49, or equivalently 40 05' 15", -104 58' 49"
ex: N 40 05.260, W 104 58.828, or equivalently 40 5.260, -104 58.828