Clojure is a dialect of Lisp, and shares with Lisp the code-as-data philosophy and a powerful macro system. Clojure is predominantly a functional programming language, and features a rich set of immutable, persistent data structures. Clojure runs on the JVM.

To get satisfactory results on Ubuntu, I suggest the following setup:

1. Remove all the java-gcj stuff that Ubuntu probably has installed:

$ sudo aptitude remove java-gcj*

2. Install the Sun Java binaries:

$ sudo aptitude install libjline-java sun-java6-jdk sun-java6-jre ant

3. Download and install git head of Clojure:

$ git clone git://github.com/richhickey/clojure.git
$ cd clojure
$ ant
$ java -cp clojure.jar clojure.main

If the last line presents you with the Clojure REPL (Read-Evaluate-Print-Loop) prompt ("user=>"), then you are have Clojure successfully running.

Next, we want to move Clojure to a more sane place (I chose /usr/local/lib/clojure):

$ cd ..
$ sudo mv clojure /usr/local/lib

Next, we want Jeffrey Chu's Clojure Extras:

$ wget http://github.com/jochu/clojure-extra/zipball/master -O clojure-extra.zip
$ unzip -p clojure-extra.zip "*clojure.conf.sample" > .clojure.conf
$ sudo unzip -p clojure-extra.zip "*clojure" > /usr/local/bin/clojure
$ sudo chmod 755 /usr/local/bin/clojure

Next, edit ~/.clojure.conf and change/uncomment the following lines:

clj=/usr/local/lib/clojure/clojure.jar
clj_ext=~/.clojure
clj_wrapper=jline.ConsoleRunner

Although I don't understand why you should need to, I had to link the jline jar file into ~/.clojure, otherwise java couldn't find it:

$ ln -s /usr/share/java/jline.jar ~/.clojure

Next, install clojure-contrib:

$ git clone git://github.com/richhickey/clojure-contrib.git
$ cd clojure-contrib
$ ant
$ cp clojure-contrib.jar ~/.clojure

At this point you should have a fully functional Clojure install. Just type "clojure" at the command prompt.