Understanding File Extensions

On your computer there are many file types. A file type is recognized by your computer by the three (or possibly 4) letter extension. Those are the three or four letters after the period in the file name. In the example "my letter.docx" .docx is the file extension.
Those extensions let your computer know what type of file it is, and what program it should use to open it.
Your computer has default programs set to open file types. These programs may have been chosen by you, or they may have been chosen by your Operating System (such as Windows) when it was first loaded. A default program is the one the computer uses to open a file unless you tell it differently.

New users of Windows 10 will notice that Windows 10 has several new programs that it really hopes you will like and it loads those as the defaults. You do not have to accept those choices unless you want to.

You may have several programs on your computer capable of opening certain file types. For example a common file extension for photo files or image files is .jpg. Your  computer may have several programs capable of opening .jpg files.
You can choose one to be the default. The  default is the one that opens that file type whenever you double-click on it, or right-click and choose "open" However at any time you can override that action by right clicking on the file and choosing "open with" from the right click menu.
That way you can override the default and choose another program when it suits you. For example on my personal computer I have several different programs that can open .jpg files.
I have one that is the default that I use when I simply want to look at my photos. If  I want to edit a photo I have a program for that so I right-click on my file and choose "open with" and choose my photo editing program just for that occasion. I have another program that I use when I want to add special-effects, and another that has printing options that I like. I right click on my file and choose "open with" and choose the program that suits my need at that moment.
In any case it's good to know you have options. You may have your own preferences, or possibly you don't like any of the programs loaded on the computer but you may find that there are some that you can purchase or download that will fill the bill.

Digital music files have several common extensions such as .wav, and .mp3. Again you can choose how you want to open these by default, but override that choice at any point. I tend to switch mostly between Windows Media Player and iTunes, but you may have other personal preferences.

Video files have extensions such as .avi, .mpg, and several more. There are any number of programs that can play videos, some with more features than others. I like VLC, a free video program that can open all manner of video files. You may have a preference for a different program. Same rule applies - choose one as the default, but be aware you can override that choice at any given time.

Aside from these fairly common file type extensions, some programs have their own proprietary file extensions. For example kindle books have the file extension .mobi
Adobe Photoshop saves it's files with the .psd extension.

If you want to send a file to someone else, you want to be sure that they are able to open that file. If they don't have the same program that you used to create the file you may be able to convert it, or a program that uses text and graphics will often allow you to save the file as a .pdf. This is an open format file extension. Anyone can download a free .pdf viewer so most people can open a file that is saved as a .pdf file.
At one point almost everyone used Adobe Reader as their PDF viewer of choice. Now there are several excellent alternatives with varying features. I use Fox-It Reader, it's also free. It's easy to update when necessary,  but doesn't update too often. Best of all it offers tabbed viewing so so you can multiple PDFs open at a time.
Once again it's personal choice. You can have multiple viewers installed. I also have Adobe Reader, and although Fox-it reader is my default, I can open a .pdf at any time with Adobe Reader with the right click, "open with" option

Knowing a little about how file extensions work will help you to to recognize file types, especially those downloaded or sent to you. Also, you can decide how you want to open your files. I've known of people who struggle along with a program that they just don't like ( for opening photos for example) unaware that they could easily choose a program that is much better suited to their needs or preferences.

And if you don't like those new Windows 10 choices? Give them a fair trial, but if in the end they are not for you, change to something else.

If ever you come across a file and you don't know what it is, or cannot open it, a Google search on its extension will usually tell you what program created it, or what basic type of file it is ( audio, video, text,  or graphic for example)

If you are interested in finding out more about file extensions and what programs they are associated a quick Google search will yield all sorts of info, but this is a good start:
http://fileinfo.com/filetypes/common

*Note 1  File Manager  is the program you use to find all your files. This is where you will choose a file to open or right click on

**Note 2 Occasionally "open with.." Does not appear when you right clicking a file. In that case try holding down the shift key while you right click.

***Note 3 To change the default program for any given file type, Right Click to choose "open with" but before you click on "open with" make sure to check the little box that says " always use this program to open this type of file" - or words to that effect. The program you choose will now be the default. If you change your mind, do the same again but choose a different program.