Direct response marketing is when you engage in some type of advertising or marketing effort which targets a market with an offer. The offer is such that response can be measured. Direct contact is made from the prospective customer to the marketer. Many B2B lead generation activities fall into the direct response category.
Direct marketing is when you target a named company, title, or individual with an offer. Contact is made from the marketer directly to the prospective customer. As with direct response, the offer can be a variety of things including attendance, discussion, communication, inquiry, etc. And response can be measured. In complex sales environments, direct marketing is closest aligned to sales and sales activities.
Hans De Keulenaer
manson 
Sergio Ferreira
davidchapman