Filenames in Windows have two basic parts. The filename itself, and its extension. Example: letter.doc. “Letter” is the filename, and “doc” the extension. The extension denotes what type of file  it is, and tells the computer what program to use to properly handle that type of file. For example, typically, files with a “doc” extension are handled by Word, the Microsoft Office document editor and word processor. It’s not hard to figure out that “doc” refers to “document”. Similarly, “txt” as an extension refers to “text” files, and so on.  Note: By default most versions of Windows will hide the extension for known types of files. So instead of seeing “letter.doc”, you might only see “letter” as a filename when looking at a list of your documents, and so forth.

But out of the files extensions list, a class stands out because of what it can do. It’s the family of extensions that allow a file to issue commands to the computer. When we talk about opening, executing a program, we’re talking about these files. Extensions that are included in this class are “exe”, “com”, “bat”, “cmd”, to name a few. If you open a “doc” file, it will show you the document. But if you open any of this class of executable files, they will execute (carry out the instructions) within it. Example: when you click on the Internet icon on your computer, the program that displays websites for you will open up in a window. If the Windows default program is the one being used for displaying websites, it is called Internet Explorer. But do you know the actual filename for it? It’s “iexplore.exe”. that’s the full filename for Internet Explorer. Because the extension is “exe”, it executes instructions when you open it. Namely, downloads data from the Internet, displays websites, sends (uploads) data to the Internet, and so forth.

So why am I babbling about all this today? Frankly, I sometimes miss the boat on how basic I need to go with my explanations to make clear why it’s not a good idea to download and open executable files from random sources (websites, etc). And on the other hand also to clarify that downloading an executable file from a website and executing (opening) it are two different things. A client of mine was trying to update one of her programs. She downloaded the executable file that was needed to update (bring to a newer version) what she was using. But she didn’t realize that downloading the file without opening it once downloaded would not perform any updates. She was wondering why she kept on being prompted to update the same program over and over! Once that was clarified and the downloaded file was opened (executed), she did not get prompted to update anymore.

One similar situation: somebody is told to install X program from a trusted website. What does “install” mean? it means getting a copy of the file from the website (download), and then executing the downloaded file, the “installer”. This is just a special type of executable program – Its instructions perform the needed steps to make the program being installed work. So sometimes users are told to install a program and given a website address where the program installer file can be found. They download the file, and happily report the program is installed (just because they downloaded the file that needs to be opened to install the program, but have not opened the file and thus have not installed anything). So, “download” and “install”: Two very different things.

Hope this helps.