Q:

What is the difference between vi and vim?

As far as I know vi is more commonly found on out-of-the-box unix systems while vim often has to be installed. Also vim stands for vi improved, but improved how?
What are the main differences?

A:

Vim tries to resemble the syntax and semantic of Vi command as much as possible. But being an "improved version", Vim adds new commands and features. It also changes the semantic of some Vi commands to better match the current expectations of its programmers.
A detailed list of changes between vim and Vi can be found using the command :help compatible in Vim.

