ERASE ~ When a VSAM data set is deleted, only its catalog entry is removed and the space is freed up for future use. However, the data is not physically deleted. Using the ERASE parameter would remove the catalog entry as well physically overwrite the data component with binary zeroes.
PURGE ~ A data set can be allocated and given a retention period using the TO and FOR parameters of the DEFINE cluster command. Such a data set cannot be deleted before the expiration date unless the PURGE parameter is used.