BB - common acronym for Bottom Bracket.
An eccentric BB is essentially a housing that sits in the eccentric BB shell and holds the BB unit in place. It can take any kind of BB but it nearly always holds a sealed/cartridge (inboard) unit.
Typically, in most BB units the spindle is fixed and does not move. But with eccentric BBs, the spindle does move. (The movement described is the actual fore and aft movement of the BB spindle in relation to the BB shell in the frame, not the circular motion of the BB spindle itself.) This movement allows the cranks, which are attached to this moving spindle, to move forwards or backwards also, thus allowing you to adjust chain tension.
There are many types of eccentric BB, but they essentially all do the same job, which is to move the BB spindle in order for you to be able to tension the chain as described above. Because of this, eccentric BBs are used in any drivetrain system where chain tension cannot be achieved by other means.
WORKSHOP TIP: They are very popular in tandems, single speed and hub gear bikes.
