The UTM grid, or known by its full name as the Universal Transverse Mercator grid, is a metric grid often encountered by backpackers, cross-country trekkers, and others who use topographical maps.
Based on the more well-known latitude/longitude grid, the Mercator grid is, to the best of my knowledge, used most often by land navigators, and probably not at all by mariners.
Many United States Geological Survey (USGS)topographical maps don't contain this useful grid, but at least have UTM tic marks in the margin, which you can connect with a ruler to draw your own grid on the map. Some newer USGS maps, however, do depict the grid.
Easy to Use
While it may take some effort to learn to use the Universal Transverse Mercator grid, once you do, it is much easier than the lat/long grid when it comes to easily identifying the coordinates of specific locations on the map. This assumes, of course, that the grid--either pre-printed on hand-drawn--appears on your map.
An Instructional Video on the UTM Grid
Embedded below is a 30-minute video I made for the purpose of helping you learn how to use the Mercator grid. If you're like I was when I was first learning this grid, you may have to study it a while before it all starts to make sense to you.
Eastings and Northings
Instead of lines of latitude and longitude, the Mercator grid lines are called "eastings" and "northings." Eastings are north-south lines, and northings are east-west lines.
From 84 Degrees North to 80 Degrees South
The Universal Transverse Mercator grid covers the globe from 84 degrees north latitude to 80 south latitude. It does not cover the poles. For polar navigation, you'll need to know the Universal Polar Stereographic (UPS) grid.