Pubsub at its core is really a simple concept. There is a store of data to which you subscribe; you can retrieve information from this data store, and when there are updates, you are told about them. While it is a simple concept, it is also a very powerful one because it's hard to think of an application that doesn't use some form of publish/subscribe arrangement, a database for example.

One of the advantages is that once the server receives this stream opener and knows that it is configured for a component on this address it responds to XMPP. Pubsub is that clients that do not need to poll the server for updates (or the "are we there yet?" syndrome) but will get pushed updates when they happen. This property of pubsub makes it ideal for building real-time systems as it reduces the load on the server from polling and ensures that data is received by clients as soon as possible.