Uh.. I know it's been 10 days :p

Let's say you have a map, and you change a value in it. Being a functional programming language, and the map being an immutable structure, you end up with two maps: the old and the new. Now, the cool part is every data-structure in clojure is designed _not_ to do this by copying the whole thing. You'll have two maps who share everything except the changed value. Same with collections, sets, trees etc. The work under the hood must be impressive...

Another nice thing (for me at least, coming from imperative programming) is there is a better difference between a variable and a value. A variable is a placeholder, and a value is ... well, a value. In clojure you treat them separately. A better explanation: http://clojure.org/state

Persistent in this case means that if FOO has value '((2 3) 4) and someone does (CONS 1 FOO) => '(1 (2 3) 4); the value of FOO does not change, but the new list shares its structure.

If you do a cons cell diagram for this it will make perfect sense.