These are long, continuously cast semis of less than 150mm square/round section. Like blooms they are cut-to-length after casting, and are reheated for rolling.
Billet is the starting point for rolling products like rebar, plain bar, merchant bar and light sections, narrow strip and wire rod. It is also used for seamless tube making.
Where ingot casting is employed, blooms are the starting point for rolling billet. And for more detail click and see wikipedia entry.