A particularly dubious service offered by some web hosting companies. Responsible web hosts will close accounts if their users are found to be involved in spam, a pyramid scheme, or other anti-social activities.
Bulletproof hosts specifically target their services towards spammers. They offer a guarantee that they won't terminate the account even if there are reports of spamming.
Damn, this guy has been sending porn spam and viagra adverts to my 10 year-old daughter but his host won't shut him down. Must be bullet-proof hosting. Where's my shotgun?