Harley got it mostly right.
Directv is digital transmission and mostly SD. The output to your TV is still analog (unless you have a HD receiver and use the digital output). Oh, and the audio output can be digital if you are using the right connections.
Cable can provide analog and digital, HD and SD.
Don't forget your Over The Air broadcasts. I receive via antenna in my attic, analog and digital broadcasts. The digital broadcasts have a mixture of SD and HD programming.
Clear as mud, eh?