General description of Twelf
This quick introduction to Twelf is aimed at people without any specific technical background. If you want more information, you can find it on the documentation page.
C code and Java code describe programs, HTML code describes graphical web pages, and Twelf code describes logical systems. Logical systems are sometimes strange to think about, because the only logical system that most people have used is basic arithmetic (addition, subtraction, division, etc.), or maybe set theory if they're particularly ambitious. Most people that use Twelf aren’t interested in using it to do basic arithmetic, because there are a great deal more interesting logical systems than just the ones that we learn about in high school.
The reason someone might want to use Twelf code to describe a logical system is that once they’ve described it, they can write more Twelf code that uses that logical system. You could use Twelf to write out a statement about basic arithmetic (for instance, “if a + b = c, then b + a = c”), and then use Twelf to write out a justification of why that statement is true (i.e. a proof). When you do so, Twelf will check your proof, making sure that what you said actually is true!
Twelf is by no means the only program you can use to do this sort of thing. ACL2, AUTOMATH, Coq, HOL, HOL Light, LEGO, Isabelle, MetaPRL, NuPRL PVS, and TPS are just a few (!) of the systems that will let you define logical systems and prove things with them. A lot of really amazing work is done using these different systems: one project at the University of Pittsburgh is using HOL Light to check a proof of the Kepler conjecture , and The Economist wrote an article about it  (unfortunately, it is no longer freely available).
But here’s where, for people that use Twelf, it gets interesting: it turns out that while basic arithmetic, set theory, and interesting logics are logical systems, programming languages are also logical systems - and Twelf has a couple of unique features that make it a great tool to use when the logical systems you are working with are programming languages...