.Rar files are a type of compressed file - a program finds ways to shrike the originals and offer smaller downloads so that the web site owner is not paying as much in bandwidth fees. In order to read them at their original size, they need to be downloaded and then un-compressed. This can be done with the original WinRAR program, as was mentioned before; however this is a paid program that one has to buy, or use the trial version for a few days. 7-Zip, however, is a free, open-source program that you can download and install that will work with many formats, including both .zip and .rar.
In order to open any of those plans you will need to install one of these two programs or a suitable alternative. .Zip is an old enough format that it is "built in" to windows to be able to read it natively, but .zip is not as efficient as .rar at packing files down in size, so the website operator has used a different format.