In this section, we'll talk about files and what software tends to open it.
Text and Word Processing
DOC & DOCX: Microsoft Word documents. Word can open DOCX, OpenOffice and LibreOffice can open DOC. Open and Libre can kind of open DOCX, but some advanced features, such as 3D text and complex gradients, will be lost.
ODT: OpenDocument Text. OpenOffice and LibreOffice use this filetype natively. Word will (begrudgingly) open it, but it will complain about the file being corrupt (the only thing corrupt around here is Microsoft).
RTF: Rich Text Format document. WordPad, Word, OpenOffice, and LibreOffice can open it.
TXT: Text file. Notepad and Wordpad can open it.
Audio
MP3: MPEG Layer-3 file. Every audio player can open it.
WAV: Microsoft PCM sample. Every audio player and audio editor can open it.
AIFF: Apple PCM sample. Every audio player and most audio editors can open it.
MID: Sequenced MIDI file. Windows Media Player on Windows 7 and below can open this, all versions of VLC media player can open it, provided a General MIDI-compatible soundfont.
FLAC: Free lossless audio compression. Sort of obscure still, but VLC media and most audio editors can open it.
Video
MP4: MPEG Layer-4 file. Every video player can open it.
MKV: Matroska Video Container file. VLC media player can open it.
MPEG: Ye olde thyme videoe filee. MPEG is still supported in Windows Media Player (for some reason).
AVI: Audiovisual Interleave. Every video player can open it.
MOV: Apple iMovie file. Most video players can open it. WMP used to have a problem with MOV, but that got ironed out in Windows 7, I think.
Images
JPG: JPEG file. Every image viewer, editor, web browser, and desktop publishing application can open this.
PNG: Portable Network Graphic file. All the same software that can open JPG can also open PNG.
GIF: An image file that can be either animated or static. Windows Image Viewer on Windows 7 for some reason can't open GIFs, relegating them to Internet Explorer. Windows 8-11's Photos app can open GIFs natively.
ICO: Icon file. Most of Windows' stock icons are packed away in DLLs, but you can edit standalone ICO files and create new ones in GIMP and Photoshop.
File compression and archives
ZIP: WinZip compressed folder. Windows can open these natively.
7Z: 7-Zip file. 7-Zip and, I think WinRAR, can open these. Only 7-Zip can save files in 7Z format.
RAR: WinRAR compressed folder. WinRAR and 7-Zip can open these, only WinRAR can save files in RAR format.
Miscellaneous and Windows system formats
EXE: Binary executable. These are the actual software programs that you run to do stuff with, like GZDoom, Mozilla Firefox, Snes9x, WordPad, and all that.
MSI: Windows installer file. Usually, installers are in EXE format, but a scattering of developers, particularly Microsoft, Mozilla, and Google use MSI. If you need to install something from MSI, just treat it like an EXE and click on it.
DLL: Dynamic link library. Software file directories are lousy with DLLs, but there's not really any need to mess with them.
CFG: Configuration file. Human-readable configuration settings that can be opened in Notepad and reset manually. Usually, there won't be any need to do that, though.
There are loads more esoteric filetypes in Windows, but these have been the kinds you'll encounter the most as a casual computer user.