I will try...
Advanced Disk - this is what you would think of as normal disk - an empty volume that you allocate for use for backups - backup images are written there as actual files. As it is Advanced disk (also know as Enterprise Disk) you can use this in Storage Lifecycle Policies. Think of it a little like NT backup - it just dumps the backup data into backup files. If you have Capacity or Enterprise Disk license you can use this as a simple backup to disk target
PureDisk De-Dupe Pool - true PureDisk is actually based on its won operating system that is Unix based - so to use PureDisk you add in a PureDisk Server that stores the backup data. This is in a complex format that is not readable in the normal sense as every piece of data being passed to it is fingerprinted and file fragments that have mathcing fingerprints do not have to be stored more than once and just a reference is stored in the PureDisk database. To use PureDisk pools your normal NetBackup Media Server is given access to it and data passes via the Media Server to the PureDisk Server (or it could be an N5020/N5030 Appliance). Effectively this uses OST to connect to the PDDO pool. Use this if you have purchased PureDisk or an Appliance and are licensed for it
MSDP - this is pretty much PurDisk but the code has been written in such a way that the Media Server itself can host the database and de-dupe disk so that the Media Server can do all of the work and not have to pass it across to a PDDO server. If you have Capacity or at least a de-dupe license you can use this on your media servers provided they meet the compatibility requirements
OST - This is usually when another vendor is involved, such as Quantum or DataDomain. Those systems have the storage and do the work but the OST plugin allows your media server to connect to them and pass the backup data to them to store. They have their own de-dupe engines etc. Use this if you have purchased another vendors appliance that uses de-dupe
Hope this all helps
