Reader Conditionals in Clojure

Correct.

To expand on this a bit more: while all three implementations are "Clojure", since Clojure is a hosted language (i.e. doesn't specify/implement its own VM) and embraces this fact, there are subtle differences between the various implementations. Some of these differences could be viewed as "bugs" and should eventually go away (e.g. cannot use macros at runtime in CLJS), but some are inherent to the platform (e.g. how do you convert a string to an integer?).

Originally, if your code was simple enough and didn't touch any of the parts that were different between the implementations, then you could simply run the same code under Clojure, ClojureScript, and ClojureCLR. If it was even mildly involved, though, you'd have to write separate implementations for each.

Then the CLJX project came along. It introduced the `#+clj` and `#+cljs` forms that could be used to specify code that should only run under Clojure or ClojureScript respectively. For Clojure v1.7.0, the goal is to implement something similar in the core language. At first the plan was to simply adopt the CLJX forms directly, but as of 1.7.0-alpha6 there is this new concept of a conditional reader.

Ultimately, as a beginning Clojurist the impact to you should be small. Eventually, if you are looking to write libraries to be used across both Clojure and ClojureScript, this will be useful to reduce the amount of duplication in your project. As you move towards the heights of advanced Clojure usage, though, this turns out to be a very powerful concept that could potentially be useful for a wide range of applications.