Procaspases are the inactive precursors to caspases.They are activated by cleaving one or two specific aspartic acids. The procaspase is split into two subunits that form a dimer. Two of these dimers combine to form the activated tetramer that is a caspase. This process is activated by already active caspases.
There are different types of procaspases. There are initiator caspases which are activated at the beginning of the proteolytic cascade. They then activate executioner procaspases. These cleave and activate other executioner procaspases as well as target proteins. These target proteins then help the process of apoptosis. Alberts et al., 2008: 1118