A small block is an engine block with usually less than 6 litres of displacement. Although there have been exceptions where the displacement has been more than 6 litres and one of them is the ever famous 427 or 7.0 litre engine.
A small block is an engine block. Pretty simple, an engine block is a machined part that holds the pistons and other parts into place. It is basically the frame of the engine. Displacement describes the total air/fuel mixture an engine can draw in during one complete engine cycle.