This is called multicasting. Standard definition pictures aren't up to the quality of HDTV, but are an improvement over our present picture. Because DTV is digital, other images, transcripts, teacher's guides, audio, etc. can be delivered to your DTV set while you are watching a program. You might be able to access the Internet and send email over DTV as well. The key is compression.
Compression allows a vast amount of data to be reduced for transmission.
In the U.S., the DTV signal is reduced to 19.4 million bits per second.