Direct marketing will be focused on a certain kind of person, a company will target a certain 'type' of person that is most likely to respond positively to the marketing material.
People can be targeted based on demographics which is age, sex, location, where they shop, what they buy, how much they spend, and additional aspects! If there is a factor that can differentiate you from anyone else then that can be used to target your product/service in a direct marketing campaign.
Mass marketing however would mean targeting everyone regardless of the possible future interest in the product/service.
A:
Direct marketing is to an extent founded on targeting the right people, however this is still quite hit and miss as data quickly becomes dated. Direct marketing techniques are now being used to sell internet products. Internet searches are sometimes refered to as "pull marketing" which is where the user is seeking information rather than mass markteing and direct marketing where you are "pushing" the message onto people and hoping that some of them convert.