The Shimano external bottom bracket has been around for ages now and is arguably the most common bottom bracket standard. It’s been superseded by various press-fit incarnations lately but the threaded, external bearing design is the easiest to work on, and offers superb performance and longevity.
It works on the same principle as a threadless headset — the cranks are two-piece, with the axle part attached permanently to the driveside crankarm. The non-driveside crankarm slides over the spindle, much like a stem would over a fork steerer. The non-driveside crank arm is clamped to the spindle to hold everything together, and a cap threads into the end to pre-load the bearings.
Shimano has updated things recently with smaller diameter cups on the XTR and XT groupsets. These are lighter and have smaller bearings, which means a different tool is required for fitting and removal. Aftermarket BBs are supplied with a plastic ring (TL-FC24) that is partnered with a regular external BB tool such as a Park Tool BBT-9 for fitting and removal.