The prepared Scout always carries a map of the area and a trail guide with him when hiking.

When planning a hike there is no substitute for a professionally printed large format map like those published by the Appalachian Mountain Club.

For hikers who need a map in a hurry, or who need a map of a specific area, there are many online sources for topographical maps and trial guides that you can download and print yourself.  There are also several good topographical map apps for your tablet or phone that use the device's built in GPS  with  downloaded maps that you can use even in places with no cell reception. The GPS will tell you  where you are on the map, at least until the battery dies or it gets wet.

There are basically three kinds of maps available online:

1. DRG, or Digital Raster Graphic maps which are scanned copies of traditional USGS topographical maps
2. Online maps based on the National Map  which can best be thought of as a seamless digital map, similar to Google maps.  Also referred to as US-topo maps
3. Custom maps and trail guides created for a specific hiking area or trail

Surprisingly, the best online maps for hikers are the DRG maps or the custom maps, not maps based on the National Map.  The reason is that the National Map does not yet contain trail information for many areas. The DRG maps, scanned from the historical printed USGS topo maps show many more trails, particularly the ones less travelled.

Here is a brief comparison of available online map resources and map apps.

The USGS  publishes a handy guide to reading topographical maps and understanding map symbols
For those interested in historical topographical maps,   MyTopo maintains a historical map archive