Often web sites make files available for download. You click on a link on the site and instead of a new web page opening you get a dialog box asking if you want to download or open the file. I would recommend that when this comes up that you should always select Save rather than open since this will place a copy of the file on your computer that you can keep (for future use to save you having to download it again) as well as allowing you to check the file for viruses before opening it.
If the web site owner is sensible then the file will be a compressed format that allows you to download the file quicker but which will require that you use a decompression program (such as winzip) to decompress the file before you can use it. With a compressed file, I recommend that you run virus check on the extracted files after you decompress them (as well as on the compressed file itself).
Files sent to you via email may also be compressed (or encoded) if the sender is sensible since this also makes for a faster send on their part and a faster receive on your part.
So how do we tell if a file is compressed (or encoded) and what do we do to decompress such files so as to make them useable on our system? There are a number of compression and encoding formats in use. The most common today is zip but there are also a lot of others. The following is a list of the compression formats that I know of to help you to recognise when a file that you have received needs to be decompressed (and in some cases that the format is incompatible with your operating system):
Encoding is also used when transmitting binary files (programs, images, and other non-text files) over the internet. What encoding does is to translate all non-text in the file into text before sending and decoding will convert it back when it is received. Your email program probably handles this for you automatically but if it doesn't your decompression software will probably do it for you. The following are encoding formats:
As you can see there are lots of compression and encoding formats. The thing is that whenever you receive an email attachment in any of these formats or download a file from the internet that is in any of these formats you need to use compression software to convert the file into a useable format.I don't know of any compression software that will handle all of these formats but there are a lot of them that you will probably never see and the more common compression software such as winzip and stuffit (on the Mac) can handle most of the more common ones.
Installing your decompression software should automatically set up all of the supported file types to open that program when you select a file of that type. If it doesn't then you can set the program association manually.
This article written by Stephen Chapman, Felgall Pty Ltd.