Garmin currently makes three Forerunner models: 101, 201 & 301. Forerunner 205 and 305 are just being added to the lineup.
The 101 and 201 models have identical functionality except that 101 cannot be connected to a computer and does not have a built-in rechargeable battery.
301 includes a heart rate monitor and has a few more features than the 101 / 201 models.
Forerunner 205 and 305 are a lot smaller than earlier versions.