A mac computer comes with program that can extract zip files or create archives, which is by default. But there is nothing given for .rar files and its extraction. You would just see an unknown file error when you try to open a .rar file in your mac.

Here is a .rar extractor for mac -
UnRarX is a Mac OS X Cocoa application that allows you to expand rar archives and restore corrupted or missing archives using par2.
It does nothing else than expanding the rar archives, and is enough for its function as one would expect a small app coming only for the rar extraction.

If the .rar file is password protected -
In order to extract password protected rar files you must follow the following procedure:

• Click on Toolbar Item labeled “Password”(a sheet should slide down from main application window)
• Enter your Password
• Click OK button
• Drag your file into the UnRarX window to begin extraction.

Check out and download the UnRarX app from here.