They are the maybe one of the most common files you will download on the internet. But do you know how to open them?
RAR files is an archive file which stores or contains some files. The reason they use it, is it add multiple files as one download. Most downloaded things today are stored in rar-files. You need a program to open those. It would recommend using "Win-RAR". Its a free program which runs smooth and fast, and helps you to easily extract the files you want. Download Winrar here
ISO files are used to store a CD/DVD into a file. A movie, or a game, or anything else that could have been stored on a CD/DVD can be stored in an iso file. Its used alot when you download stuff, and its easy to open it. With "Daemoontools Lite", you can open the ISO file like in a virtual CD/DVD drive. This way, you can launch the iso file like if it was a CD/DVD in a real room. Download Daemoontools Lite
Good luck with. I also have an video explaining this: