A grid is a map coordinating system that uses square zones to relate positioning measurements for establishing points of reference. Some systems will have (multiple) preinstalled grids of countries built into the GPS OS.

MGRS - Military grid reference system.
OSGB - British national grid reference system.
USNG - United States National Grid.
UTM - Universal Transverse Mercator coordinate system.
WGS - World Geodetic System.