Peter Hinely wrote one of the more concise descriptions of Dylan:
Dylan is an advanced, object-oriented, dynamic language which supports the rapid development of programs. When needed, the programmer can later optimize [his or her] programs for more efficient execution by supplying type information to the compiler. Nearly all entities in Dylan (including functions, classes, and basic data types such as integers) are first class objects. Additionally Dylan supports multiple inheritance, polymorphism, multiple dispatch, keyword arguments, object introspection, and many other advanced features...
Some code examples can give a quick feel for the language.
The Dylan Hackers maintains two Dylan implementations: Gwydion Dylan and Open Dylan.
Gwydion Dylan is a Dylan-to-C compiler originally created by a team at Carnegie Mellon University. Our current release is a technology preview, suitable for learning the Dylan programming language and building command-line applications. Gwydion Dylan produces high-performance code, but has a number of limitations that make it difficult to use for the beginner. Gwydion is highly portable, with binaries available for many versions of Linux, FreeBSD, Mac OS X, Cygwin, etc.[ Download | › Why "Gwydion"? | › Gwydion Maintainers | » Gwydion at CMU | Dylan History ]
Open Dylan compiles to native code and has a full-featured IDE including an incremental development mode, browsing of runtime objects, remote debugging, etc. Open Dylan currently only runs on the x86 platform and the IDE does not yet run on the Linux version. Open Dylan is in many ways a mature implementation. If you are new to the language, choose Open Dylan if you can.[ Download | Dylan History ]