A file extension is the suffix after the period (".") in a filename (e.g., ".txt" in "report.txt"). It identifies the file format or type, signaling to the operating system and applications which program should be used to open or interpret the file. While the extension itself doesn't dictate the file's internal structure (which is determined by its content), it's the primary method systems use to associate files with software. MIME types, a more precise internal identifier, often work in tandem with extensions.

Common examples include ".docx" for Microsoft Word documents and ".jpg" for compressed image files. When you double-click a ".pdf" file, your computer typically launches Adobe Acrobat Reader because it's associated with that extension. Web browsers use extensions like ".html" or ".htm" to know they should render the content as a webpage, while servers use them to determine appropriate MIME types for delivering content correctly.

File extensions provide essential user visibility and system functionality for file handling. However, they can be misleading or changed, creating security risks if a harmful file (e.g., malware) disguises itself as a safe extension like ".txt". Future developments involve smarter systems using content-based detection alongside extensions, but extensions remain fundamental for user identification and software compatibility on most platforms despite their potential for misuse.