A file extension is a suffix at the end of a filename that indicates the file type and format. It usually consists of three or four letters following a period, such as .txt for text files or .jpg for image files. File extensions help operating systems and software programs understand how to open and process the file. Different file extensions correspond to different types of data and applications. For example, .docx is used for Microsoft Word documents, while .mp3 is used for audio files. Recognizing file extensions is essential for managing files effectively and ensuring compatibility with the right software.