Even if you've never heard the term, you're part of the BBBH community if you're a guy who enjoys giving and/or taking loads of cum in the ass (aka "breeding"). So #BBBH is a group of men (not women) who are bound by the exchange of cum. Using condoms, pulling out before cumming, giving/taking oral loads, getting/receiving cum facials, etc. are not part of #BBBH, though some BBBH members enjoy those in addition to giving/taking anal loads.
In February of 2010 barebackers on Twitter decided they needed a short, unique hashtag for the community. #BBBH was born at that point. For a more complete history check out one of the founder's recollection of how it all started.
When BBBH.com was purchased iBlastInside threatened a lawsuit if a site was put on the domain. The threat was mostly without legal merit but "Bareback Bunkhouse" is used to make sure this site is not "confusingly similar" to a site he put up a few years ago that never got much traction. BBBH.com members can opt for "Bareback Brotherhood" branding if they prefer.
While many BBBH members are HIV positive, barebacking can be done relatively safely these days. The benchmark is condom use which is 10 to 20 times safer than unprotected sex. With that in mind:
This means sex with poz guys is often safer than sex with guys who say they're neg since about 1/3rd of poz guys don't know they're poz and falsely tell people they're neg when they're actually poz and quite contageous.
To put the point simply, anti-retroviral medication is more effective than condoms at stopping HIV – which means barebacking can be safer than having "safe sex" with rubbers. Either person can take them - the poz guy or the neg guy. For this reason we encourage all neg guys who are having regular anonymous bareback sex to get tested regularly and talk to their doctors about taking PrEP.
For more information on HIV risks, check out the HIV Risk & Risk Reduction section on Breeding Zone.
PrEP stands for Pre-Exposure Prophalaxis. In other words, it's when HIV negative guys take particular types of HIV drugs that fight HIV infeciton when/if they're exposed. Only one drug has been approved for PrEP so far - Truvada. It was chosen because it has a long track record of safety. Nevertheless some people do experience side-effects that are sometimes quite serious, so it's critical that people on PrEP see their doctors every three months for checkups.
For more information on PrEP, check out the PrEP discussion section on Breeding Zone.