Files have two parts of their names. Often people only see the first part and think of that as the file name. However almost all files have a second part to their name which is the extension. The extension identifies what program the file is designed to be used with or the type of file.
You may have heard that every file in a folder must have a different name but sometimes noticed you had several files with the same name and wondered about the rule. What happened is the extensions on files that the computer knows what program they are associated with have hidden the extension. That means the files actually have different names.
Extension is separated from the first part of the file name by a period. So, a file with an extension might be dwight-pic.jpg
Common extensions on files are .jpg, .doc. .docx, .exe, .mp4, etc. These extensions tell what type files they are. .jpg extension shows it is a picture, .doc and .docx are file extensions for documents usually made in Microsoft Word, .exe is a program file that can be run or executed, and .mp4 is a video file. There are thousands of extensions and normally they are three characters long, which dates to DOS. However, in Linux — and actually in Windows also but seldom done — the extensions can be longer.
Windows by default hides extensions that it knows what program is associated with them, which is almost all the files that will be on most people’s computers. You can go in Windows and change the setting, so you always see extensions. Personally, I am an advocate of showing all extensions as there is no doubt exactly what the name is and for you to know what it is used for. However, Microsoft thinks it clutters and confuses the average person. To change so you see the extensions (and do opposite if want to hide) open File Explorer (either from the task bar (shows a folder icon) or the Start Menu. Then at top choose View to open the View menu/ribbon. Then to the right side you will see an option File Name Extensions. Checking the box beside it will reveal the extensions; unchecking it hides known extensions.