Birds of a Feather sessions are informal meet-ups where attendees can group together based on a shared interest to carry out discussions without any pre-planned agenda. These are typically organised in the week before the conference or at the conference, as interest develops around specific topics.
As a few examples, there may be a key-signing BoF where those who use public key encryption can get together, meet each other and offer to sign each other's keys. There may be a networking BoF where people are encouraged to meet each other and exchange business details. Another BoF may enable a number of people who are involved in project, or subscribed to a mailing list, to meet each other face to face.