^^ What Brian said. I'd also add this:

Direct Response (Marketing) or DR(M) is an umbrella term.

There is DR...

-Advertising
-Mail
-Selling
-Radio
-TV

You could probably use a blimp or something to with a special deal.

Direct response is anything that goes directly to the consumer to elicit a response.

I love Bob Stone's definition from Successful Direct Marketing Methods, 7th Ed.:

"Direct marketing is the interactive use of advertising media, to stimulate an (immediate) behavior modification in such a way that this behavior can be tracked, recorded, analyzed, and stored on a database for future retrieval and use." - Ch. 1, page 5 (Direct Marketing Defined).

Cheers,

Angel