Crumbs works in two basic modes - dropping a crumb, and finding crumbs you dropped. A "Crumb" can be thought of as a waypoint.
When you start the app, the inital crumb is dropped, and is alway your "start". You are given the option to record this activity as either a "walk" or a "run". If recording you get more info as far as time, distance, pace, speed etc.
The main page then shows crumb counts (you can drop up to 50), your current lat/lon, and direction, and some of the info available when recording. You are also given the option to drop a crumb or to start finding the crumbs. When you are finding crumbs, you are shown the lat/lon of the last crumb dropped, as well as the direction and distance "as the crow flies" to that crumb.