All are related to deduplication technology. Simply put, these are all used to compress original data down further than even most file compression utilities by comparing data "blocks" (a single file can be made up of multiple blocks, and block sizes can be fixed or variable in size). If the block has been seen before, it isn't stored a second time, but a pointer to the existing block is stored in the file ssytems index. If the block is changed and two files point to the same block, the changed block will be written to disk and the two files will then point to their own separate blocks.
Puredisk - A general purpose file storage system that can be used on a customer's existing disk-based storage hardware. Data may be written to this storage by any application that you grant. An example would be a file server where there are a lot of files that may be similar and contain similar blocks of data. It may also be used with NetBackup to store backup images. It used your own separately purchsed disk storage hardware. This is a completely separate purchase from NetBackup and is licensed differently.
MSDP - This is a narrow-purpose file storage system, used specifically by NetBackup. It is based on PureDisk, but it is a completely different implementation and is used solely by NetBackup for storing backup images, no other applications may access this storage space. It also uses your own supplied storage hardware. This is an additionally licensed option with NetBackup.
Open Storage (aka OST) - This is an interface between NetBackup and 3rd party deduplicating hardware, like Data Domain, Quantum DXI, etc. These devices have features that allow them to duplicate data from one of their devices to another as soon as a file is closed by the operating system, completely outside of anything NetBackup would know about. Without OST, this would make the copy of the data at the other device invisible, and therefore useless, to NetBackup because the catalog would not know where the data is located. They are fast and generally more efficient than the above two software-based solutions, but they are expensive. As already stated, this uses proprietary hardware from a storage vendor that supports the OST API. This is an additionally licensed option with NetBackup.
