File extensions indicate the file type and by default, Windows hides most known extensions. Much more about file types here.
A typical e-mail attachment may be .doc (Word documents), .jpg (photos/images) and so on. These are known file types and if the attachment is opened, Windows will run the associated application.
.exe, .bat and .com (just to name a few) are also known file type but rather than opening a specific application, files with these extensions are in themselves applications and when opened they do whatever they are programmed to do.
Viruses writers know about hidden extensions and exploit this problem, e.g., an attachment named photo.jpg.exe may appear as only photo or photo.jpg when it is actually an executable file. If opened Windows will run the executable file directly launching the virus.
To view all file extensions, open the Windows explorer (My Computer) then from the Tools menu, select Options then click the View tab. There you will find the option to hide known extensions, simply uncheck that option.