Vim is a highly configurable text editor built to enable efficient text editing. It is an improved version of the vi editor distributed with most UNIX systems.
Vim is often called a "programmer's editor," and so useful for programming that many consider it an entire IDE. It's not just for programmers, though. Vim is perfect for all kinds of text editing, from composing email to editing configuration files.
Despite what the above comic suggests, Vim can be configured to work in a very simple (Notepad-like) way, called evim or Easy Vim.
What Vim Is Not?
Vim isn't an editor designed to hold its users' hands. It is a tool, the use of which must be learned.
Vim isn't a word processor. Although it can display text with various forms of highlighting and formatting, it isn't there to provide WYSIWYG editing of typeset documents. (It is great for editing TeX, though.)
Vim is charityware. Its license is GPL-compatible, so it's distributed freely, but we ask that if you find it useful you make a donation to help children in Uganda through the ICCF. The full license text can be found in the documentation. Much more information about charityware on Charityware.info.
Vim in Six Kilobytes
If all of this information is overwhelming, try a smaller dose. We can expound the wonders of vim in just six kilobytes -- and in more languages than you can shake a stick at!
(Text from vim.org, image from wikipedia)