Shared memory with Persistent data structures
The Clojure data structures are immutable, so they initially seem similar to constants rather than variables. Once a collection is created, it cannot be changed. Any functions that run on a collection do not change the collection, instead they return a new collection with the respective changes.
Creating a new collection each time may seem inefficient, however, the persistent collections use a sharing model. When a new collection is created, it links to all the relevant elements of the original collection and adds any new elements.
Read the InfoQ article on An In-Depth Look at Clojure Collections.