Vim stands for Vi IMproved. Vim is a configurable text editor. It is an improved version of the vi editor.
From the Vim website:
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.
The most confusing aspect of Vim (and vi) is that it is a modal editor, meaning that it has different modes. In Normal mode,
dd would delete a line, but in Insert mode,
dd simply inserts dd into the text.