There are a variety of applications on Windows to open .RAR files, and while the default choice is WinRAR by the developers of the .RAR file format, it’s not free. If you need to create files, that’s your best bet, but if you just need to extract them, you’ll want to use the free and open source 7-Zip application instead.
Mac OSX
In Mac OSX there are not as many choices as on the more popular Windows platform, however there are still a few choices, like the free app “The Unarchiver“, which has great support for multi-part archive files. Once installed, you can launch the app to associate file types with the app:
The way to extract your archives on a Mac is to simply double-click on the file. The Unarchiver will create a folder with the same name as the archive and extract its contents into the folder.