.min.js is the minified version of the javascript i.e. .js
When .min.js is used
It is primarily used while the website is being launched in order to reduce the load for the particular website.
What .min.js contain
All the long variable names and function names are converted into shorter ones during the process of minifying. As consequence it would reduce the size of that particular file, so it would less the loading speed of the website.
