zip file

a friend has emailed me a Zip file. How do i open a Zip file? What is a .Zip file anyway?

Zip files are compressed archives. The term "Archive" in this context means that the file may contain one or more files, or even folders, much like folders on your hard disk. "Compressed" means that mathematical techniques have been used to reduce the space needed to store the files, so that the size of the archive may be much smaller than the size of the files it contains.
If you're using Windows XP, then your operating system has built-in support for Zip files. They are described as "Compressed (zipped) folders." Just double-click on a Zip file and it will open in Explorer, just like a folder. Double-click on a file inside the compressed folder and you may be able to open that too (but really, Windows has extracted a copy of the file to another location and is opening that, as described above.) But if you right-click on items in a compressed (zipped) folder, you'll notice that the same options don't appear on the pop-up menu. This shows that it is not a real folder.
If you're using Windows 95, Windows 98, Windows Me, Windows NT or Windows 2000 then you'll need to install a special application to open Zip files. The application that is usually recommended to use is WinZip. WinZip became popular because it was the first ever Zip / Unzip utility for Windows.