I think it includes any unasked-for bulk mailings about anything, as well as advertising ones wanting you to buy something. Ones wanting you to visit their website, purely so that they will get lots of clicks and move up the Google popularity list, exhortations to repent and mend your ways and go to church, petitions to sign, warnings that your laptop will die horribly unless you click on a link so that the writer can fix it for you, notifications that a friend is stranded in Outer Mongolia and needs you to wire £5,000 cash to him in the next half-hour to prevent him being thrown into jail for vagrancy. That kind of thing.