What's the difference between a digital adapter and a comcast box?

Hi,
I was told that Comcast was going to be going all digital soon and that I could get up to 3 digital adapters for free for my TV's or pay for a comcast box. What's the difference between the two?

A DTA (Digital Transport Adaptor) is a very small box designed to simply convert digital cable to analog for an analog TV. It doesn't have guide data and may not work for all tiers of service. The digital STBs give an Electronic Program Guide, access to all channels, and if included in your package, access to video on demand service. If you have an HDTV you will need a HD box if you use it for anything other than "limited basic" channels. Eventually there is a good chance that a STB (or if your lucky a DTA) will be required for all channels on all TVs since the cable industry has requested, and will probably receive, permission from the FCC to encrypt all channels.

From the Comcast site: http://customer.comcast.com/help-and...gital-adapter/