What is a ZIP file?

A ZIP file is a computer file in which the contents of one or more files have been compressed for transmission. 

How do I open a ZIP file? 
Mac OS X, Windows Vista, Windows 7 and Windows 8 all have a pre-installed unzip feature. Once you’ve downloaded the ZIP file, simply click on it to view its contents.

If you have Windows XP, follow these steps: Right-click on the ZIP folder.Click on "Extract all" from the panel that opens. The system will take you through a short extraction process via a step-by-step "wizard."The wizard will ask you to choose a destination (the folder where you want to keep the unzipped files). Once the files are extracted from the ZIP folder, you can  view them in their new destination. 

Still didn’t work?
On a Mac, hold down the control key and click the ZIP file icon. Choose "open with" and then select "archive utility." The utility will extract the ZIP files and you can find them in the same folder you downloaded them to.