Vim is an open-source text editor with a steep learning curve that leads to high efficiency in many tasks related to text edition. If you wish to start with vim, try to run “vimtutor”, which will teach you basic commands.
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.