a data set that is divided into partitions, called members (It contains a list of its members, called a directory. The system uses the directory to locate a program in the library.)

a single data set which is subdivided into a small (and limited) number of "virtual data sets" called members

A collection of data known as members. All members of a PDS must have the same logical record length. A PDS member is identified by at least 4 levels of qualifiers in the name. The first three qualifiers identify the PDS. Qualifiers are separated by periods within the name. Has some similarity to a directory in personal computer systems and is often re ferred to as a library.