The way it works: each track point stores the latitude, longitude, altitude, and time. You can set the unit to record a point at a chosen distance interval, time interval, or "automatic" with five choices from least often to most often. On automatic, it watches how far you have deviated from a straight line connecting the last two points, which results in more closely spaced track points on curves.
Ron's comment about the altimeter is important. All GPS receivers can read altitude from the satellites with typical error of < 100 feet, 95% confidence. Some units also contain a barometric altimeter, which gets the error down to perhaps < 20 feet. If more precise profiles are important to you, then the extra cost and small extra battery drain may be worthwhile.
I'd like to recommend http://gpsinformation.net/
where you can find a wealth of information about consumer GPS. Jack's two lead articles,"My First GPS for Hiking" and "My First GPS for Automobile Navigation" are excellent starting points. You can hybridize them for biking.