A file extension in Windows is a suffix at the end of a filename that indicates the type of file and associates it with a specific program that can open or manage it. It typically consists of a period followed by a few letters or numbers. File extensions help the operating system understand what kind of data is contained in the file and which application should be used to open it.

Examples of Common File Extensions

1. Text Files
   • .txt – Plain text file
   • .doc – Microsoft Word document
   • .docx – Microsoft Word Open XML document
   • .pdf – Portable Document Format file
2. Image Files
   • .jpg or .jpeg – JPEG image
   • .png – Portable Network Graphics image
   • .gif – Graphics Interchange Format image
   • .bmp – Bitmap image
3. Audio Files
   • .mp3 – MP3 audio file
   • .wav – Waveform Audio file
   • .aac – Advanced Audio Coding file
4. Video Files
   • .mp4 – MPEG-4 video file
   • .avi – Audio Video Interleave file
   • .mkv – Matroska Video file
5. Compressed Files
   • .zip – ZIP compressed file
   • .rar – RAR compressed file
   • .7z – 7-Zip compressed file
6. Executable Files
   • .exe – Executable file
   • .bat – Batch file
   • .msi – Microsoft Installer file
7. Data Files
   • .csv – Comma-separated values file
   • .xls – Microsoft Excel spreadsheet
   • .xlsx – Microsoft Excel Open XML spreadsheet
   • .json – JavaScript Object Notation file
   • .xml – Extensible Markup Language file
8. System Files
   • .sys – System file
   • .dll – Dynamic Link Library file
   • .ini – Initialization file

These are just a few examples, and there are many other file extensions for different types of files and applications.