1. What is a ZIP file
ZIP is a popular archive format widely using in Internet. Like other archives, ZIP files are data containers, they store one or several files in the compressed form. After you downloaded ZIP file, you need to unpack its contents in order to use it.
2. How to handle ZIP files
WinRAR provides the complete support for ZIP files, so you may both create and unpack them. If you installed WinRAR on your computer and downloaded ZIP file from Internet, you may double click on ZIP file icon to open it in WinRAR, select all files, press "Extract To" button, enter a destination path and press "OK". Another way is to click on the ZIP file in Explorer using the right mouse button. If you enabled "Shell integration" option when installing WinRAR, the file context menu will contain "Extract to ..." item.
3. ZIP versus RAR
The main advantage of ZIP format is its popularity. ZIP file format was developed a long time ago and perhaps it is still the most known archive type, so a large part of Internet archives are ZIP files. But many important RAR format features (recovery records, Unicode names, strong AES encryption, etc.) are missing in ZIP. RAR archives usually provide a noticeably higher compression ratio than ZIP file format.