To achieve the higher capacity, TDK added two additional recording layers to the disc to take it up to four layers. Blu-ray Disc stores 25GB on each recording layer and the standard currently includes single layer and dual layer versions.More info at MacWorld
TDK presents 100GB Blu-ray disc

TDK unveiled a prototype of a Blu-ray disc with double the capacity and double the write speed of the current Blu-ray discs. The disc stores 100GB and can record data at 72Mbps.