There are several types of subs used within the industry which may vary by manufacturer, the most common being:
The bit sub connects BHA components to the bit and is often bored for a float sub.
A short sub assembly is used to enable the connection of two components with differing thread types or sizes.
A float sub has a body which contains a one-way check valve to prevent drilling mud and cuttings flowing back up the drill string.
The lift sub is a solid device useful for picking up slick components at the surface.