MVS Datasets can be broadly classified into Physical Sequential (PS) datasets, Partitioned Datasets (PDS) and VSAM Datasets. A Physical Sequential datasets can be compared to files, where the data is stored as records. The data can be a source code or application data. PDS can be compared to a folder, which has different members that contain data. Here again, each member can have a source code or application data as records.These are the only ones that can be allocated using ISPF/PDF.
VSAM datasets are special access method datasets that contain business application data. These datasets can be created to contain entry sequenced or key sequenced data as required by the application. These datasets are maintained by VSAM (Virtual Storage Access Method) and cannot be directly accessed from MVS. IDCAMS utility is used to work with these datasets.
Hope this information will give you an insight into Datasets.