Questions relating to determining your location or the location of a QSO based on one of the various systems of measurement (lat/lon, GPS coordinates, ARRL section, CQ Zones, ITU Zones, Maidenhead Grid, etc).
Location is one of those important pieces of information that is commonly exchanged between radio amateurs during a QSO. However, there are many systems in place to describe your (or your contact's) location depending on the situation. This article attempts to be a primer on the most common ones.
Lat/Lon—The old standby, the earth is divided into 360° (+- 180°) starting at the Royal Observatory in Greenich England as the Prime Meridian (or 0°). West from here the Longitude is positive, East from here, negative. for the North/South dimension, the Earth is divided into 180° (+-90°) starting at the equator. North of the equator, the value is positive, south of the equator, it's negative.
Both values can be further divided into minutes, seconds and fractions of seconds for higher resolution.
GPS Coordinates— Uses a high resolution model of the Earth and precise time measurements to give highly accurate Latitude/Longitude figures as will as altitude.
ARRL Section—The ARRL has divided the US and Canada (I imagine in cooperation with RAC) into sections along state/province lines. Populous states/provinces are split into more sections as needed. There are (currently) 86 sections. You can find a nice map here
CQ Zones—CQ Amateur Radio Magazine has divided the Earth into 40 zones for the purposes of their contests. You can find a complete description of them here.
ITU Zones—The ITU (International Telecommunication Union) is the United Nations specialized agency for information and communication technologies. They have divided the Earth broadly into 3 Regions and further divided down to 90 Zones. You can find good maps here.
Grid Square—(Maidenhead Locator System) was conceived as a simple way to describe your general location anywhere on the Earth in a way that's suitable for easy communication over the air.
The Maidenhead Locator System (named after the town near London where it was conceived by a meeting of European VHF managers in 1980), a grid square measures 1° latitude by 2° longitude and measures approximately 70 × 100 miles in the continental US. A grid square is indicated by two letters (the field) and two numbers (the square), as in FN31, the grid square within which W1AW, ARRL's Maxim Memorial Station, resides.
Occasionally you will need to use a subsquare for more precise location. Each subsquare is designated by the addition of two letters after the grid square, as FN44ig. These more precise locators are used as part of the exchange in the 10-GHz contest. They measure 2.5 minutes latitude by 5 minutes longitude, roughly corresponding to 3 × 4 miles in the continental US.
A nice article in the January 1989 QST describes the system and conversion between lat/lon and grid squares: Conversion Between Geodetic and Grid Locator Systems. This website: Grid Square Locator Map will determine your grid square location based on either your address or call sign or is you give it a grid square (either 4 digit or 6 digit), it will show you on a map the area that square encompasses.