It's a bit like Garmin Connect except better - just aimed at cyclists. You upload your spin data from your Garmin (or Android or iPhone) and you see the usual - distance time HR etc..
But the interesting thing about Strava is that it takes hilly segments out of your spin (or you can create the segments yourself) and shows you the gradient, your time, HR for the segment etc.. The really fun thing is that when someone else comes along and cycles the same route, the clever Strava goblins take their data and compare it. And whoever is fastest is King of the Mountains
so for example here is an effort of mine from the Lower Car Park to the Black Banks
http://app.strava.com/segments/859269
If everyone who has a GPS was using Strava we could have some right craic and see who really is King of the Mountains...