PS: Physical sequential file, or u can call it a flat file.
PDS: Partioned Data Set.
PDS contains member inside it, whereas PS can not.
Another difference is that During the creation of PS u dont need to mention Directory Block parameter but for PDS it is necessary.
Physical sequential data sets are for storing and retrieving data in sequentaial order.
Partitioned data sets are collection of members (may be sources,JCLs,procedure etc) whose members can be processed independently.