To put it another way, the difference is push vs. pull. A catalogue is a pull offering: the person looking at it is doing so by choice, because they're interested in what it offers and want to buy something (or at least window shop). An online ad is a push offering: it's presented to people who did not choose to see it, are not interested in it, and just wish it would go away and let them get on with what they're actually trying to do. Pull advertising is (usually) acceptable. Push advertising is not.