Clojure is a Lisp family, dynamic, functional programming language targeting the Java Virtual Machine, with excellent support for concurrent programming. Rich Hickey, Clojure author, prepared a few impressive presentations about the language, which are definitely worth watching. They are not only about Clojure itself, but also give a deep insight into computer language design and concurrent programming in general.
There have been some attempts to compare Clojure with Erlang, although both of the languages address different problem classes. Erlang has strong telecommunication background - starting with the language name itself. It features its own, highly efficient virtual machine, which allows spawning hundreds of thousands of processes and soft real-time performance. Most of its syntax comes from Prolog, since initially Erlang emerged as its dialect - everyone who programmed in Prolog before (like me) will feel at home with Erlang. Finally, Erlang supports programming model based on transparent distribution, which means that you can spawn processes throughout one or several machines without making any changes to the software. Processes (also called actors) share no common memory or data and communicate purely through message passing.
Clojure does not provide its own virtual machine with ability to handle millions of soft realtime processes. Instead, it compiles directly to JVM bytecode, which immediately gives you access to all Java classes and to thousands of libraries written for the Java platform. Clojure is based on Lisp, which is much more popular for programming real-life applications than Prolog. Therefore, despite of its (Lots (of Irritating (and Superfluous (Parentheses)))) syntax, Clojure may be faster to learn and adapt by regular software developers than Erlang. As regards concurrency, Clojure does not support distributed programming. Instead, it uses Software Transactional Memory model. Clojure agents, unlike Erlang actors, use actions instead of message passing to change their state. Actions can be committed inside transactions, which allows for example to read the state of all agents within one transaction, and this way obtain a consistent snapshot of a whole system (pretty tough to achieve in distributed enviornment).
Erlang has a 20-year success story in telecommunication industry and is especially suitable for building highly scalable, fault tolerant, distributed application clusters that are expected to work under very heavy load. It is also ready to work on multiprocessor machines out of the box. However, when finally multi-core CPUs became standard in home computers, Erlang did not sweep away other anachronic programming languages, despite its obvious advantages. I think it is because most home applications don't need many of the features Erlang has to offer. Users don't need 99,98% uptime, as they usually turn off their machines overnight. They don't need soft real-time processing, since they can easily tolerate a few second delay in data processing. Finally, they don't need high scalability, as very few of them are geeks who connect their machines in clusters (who needs it anyway, if you have more and more cores in one box?). As Rich Hickey, Clojure author, said in his discussion on STM:
"Scalability is not a universal problem - some systems need to handle thousands of simultaneous connections - most don't, some need to leverage hundreds of CPUs in a single application - most don't."
Clojure seems to meet the needs of desktop programmers slightly better than Erlang. It provides easy concurrency and all qualities of functional programming, allows you to take advantage of vast set of Java libraries, and lets you to run your software without modification on every platform for which the Java runtime exists. Erlang is unbeatable on server platform, but leaves a lot of space for its desktop counterparts. Clojure is just the first of them. I am sure there will be more.