This is a great fitness app. Runners, Cyclists, Swimmers etc. RK will record your workouts, track your progress and gives you some neat features along the way. If you're a Fitbit user or have a Withings scale, it will collect that data as well.
Runkeeper is the best running app I've used so far. I particularly like the audio cues it gives you (e.g. every 1/2 mile it can tell you your average pace, split pace, distance covered, etc.) and the solid integration with the Runkeeper.com website. I'd recommend it to anyone who's looking for a great way to track their training runs.
It records stats about your running. Speed, distance, ties it together with maps and elevation. Can be used for things other than running, such as bicycling.