What’s The Difference Between The Garmin Forerunner 205, 305, And 405?

I need a gadget like this for my outside runs, and from research it seems like Garmin is king of this world. It appears that they’ve got three different models, the 205, 305, and 405. The only difference I saw was that the 205 costs a $100 less than the 305 and 405. So, what’s the difference between these three models?