DTA stands for Digital Transport Adapter. Comcast also refers to them simply as "digital adapters." It's an inexpensive and very basic digital cable device that allows you to watch all Limited Basic and Expanded Service channels that were migrated from analog to digital (Usually the first 99 channels). DTAs will not receive premium channels such as HBO, Showtime, etc. Consult your local Comcast center for an applicable channel lineup card for details.
The DTA does not offer any advanced features such as OnDemand or Pay-per-view or even an on-screen guide. DTAs are intended for legacy analog TV sets, as such they only feature an RF coaxial output that transmits a video signal on channel 3 or 4 (switchable), in this regard it is like a VCR or the old analog cable boxes used in the '80s and '90s.
For Limited Basic users Comcast is offering the first two DTAs for free and then charging $1.99 (plus taxes and fees) per DTA after the first two. This offer may vary by region and if/when encryption of all channels is being implemented, so please check with your local Comcast office. You should also keep in mind that usually Comcast doesn't charge anything for the first full fledged digital set-top box if you subscribe to the Digital Starter or above package, further, Comcast now (as of early 2013) charges $1.99/month for any and all DTAs if customers are on Digital Starter or higher.
More information is available here: http://customer.comcast.com/help-and-support/cable-tv/limited-basic-encryption/
Does the DTA have a guide?
Not currently, the main purpose of a DTA is to be an inexpensive one-way digital cable device that allows people who used to have analog cable direct into the back of their TV to continue to use whatever legacy TVs they might have without needing to pay for a full blown set-top box.
There is a DTA/HD-DTA guide offered by Rovi (a TV guide software vendor), and Comcast will be starting trials of the guide software in late-2014, early-2015. There is a FAQ available here: http://customer.comcast.com/help-and-support/cable-tv/dta-guide/
More about HD DTAs
In 2013 Comcast, begun to offer an HD capable version of a DTA for Limited Basic subscribers who would like access to the HD versions of their channels. It costs the same $1.99/month. Like the standard definition version, it is a simple small black box, in addition to an RF coaxial output (for legacy TVs), it features an HDMI connection. For Limited Basic users there are no other fees. For those on higher-up plans it will require the addition of the HD Technology fee ($9.95) if it is not already on your account.
On the HDMI setup menu, there is an aspect ratio option for 4:3, 16:9 & AUTO, there is also a setting for the standard output resolutions (480i/p, 720p, 1080i) plus an AUTO setting - which the DTA was defaulted to. The AUTO setting is for legacy users and compatibility sake, AUTO seems to downconvert every channel (HD included) to 4:3 and 480i. You must explicitly set it to 16:9 and an HD resolution to get an HD picture.
More information on the HD DTA can be found at the links below:
Feedback received on this FAQ entry: