Java - a host platform for Clojure
You will need to have a Java Runtime Edition (usually installed on most computers by default) to run any Clojure applications. Version 8 is recommended (although version 6 & 7 should work).
To test if you have Java on your computer, open a command line window and run the command
Installing the Java Runtime Edition
Download and install the latest Oracle Java SDK (version 1.8 at time of writing).
The OpenJDK is available as a package on Ubuntu and can be installed via the Ubuntu software center or via the command line:
sudo apt-get install openjdk-8-jre
Why is Java Required
Clojure was designed as a hosted language, which means it is developed and run on top of Java's Virtual Machine (JVM). However, its not neccessary to learn the Java language to use Clojure.
Clojure is compiled into Java bytecode when you evaluate the code. This compilation happens in the background so you dont usually see it happening. For example, if you are using the Clojure REPL then each time you evaluate an expression it is compiled into Java bytecode and then injected into the running REPL and the results are then returned. This all happens pretty instantaneously.
Most of the current Clojure tooling was developed for Clojure on the JVM, for example Leiningen.
As Clojure runs on Java you can also use all the other libraries that run on the Java Virtual machine, regardless of whether those libraries were written in Java, Clojure, Scala, JRuby, jython, Groovy, etc.