The blockchain ledger is stored as a physical file, and it contains linked blocks, each of which consists of a set of transactions. Also, every state change is stored in those blocks. In contrast, the world state pertains to the current state of every asset because that is what applications exactly need.
The world state is implemented as a database (CouchDB is one of the best options). There is a simpler one available but that one offers much less in terms of querying capability. Of course, this means that the world state can be easily recreated from the ledger at any point in time. when a new peer gets added to a channel, its own world state is created from the ledger.