The bottom bracket is a component of a bicycle that connects the crankset (the part of the bike that includes the pedals and chain rings) to the frame. It is located at the bottom of the frame, between the seat tube and the chain stays, and is responsible for allowing the cranks to rotate smoothly and efficiently.
There are several different types of bottom brackets, including:
External Bottom Brackets: As their name suggests, external bottom brackets sit external to the frame and are usually either pressed or threaded into the bottom bracket shell. Popular styles of external bottom brackets include T47, BSA, GXP and DUB.
Press-fit: This type of bottom bracket does not use threads to attach to the frame. Instead, it is pressed directly into the frame and held in place with a press fit. It is considered to be more lightweight and efficient than other types of bottom brackets.
Cartridge: Cartridge bottom brackets are all-in-one bottom bracket standards that are typically designed to thread directly into a bike's frame. Popular cartridge-style bottom brackets are typically used on more affordable or classic bikes, including ISIS, Octalink and Square Taper.
When choosing a bottom bracket, it's important to consider the type of frame and crankset you have, as they need to be compatible.