A text file can have the extension .TXT or .DOC or .WPD. (This stands for text file, document file and WordPerfect document.) These are generally your text documents.  You can change your documents with one of your typing programs, or word processors. You can delete these documents when you don't need them anymore.

Programs have the extension .BAT or .COM or .EXE.  If you type the names of these files, a program will start to run. (The extensions stand for batch file, computer file, or executable file.) You can easily change a batch file, but NEVER mess with the other two!

The assisting files are necessary for the program to run.  They have valuable data.  Sometimes you can change this data. All the files that come with the programs are probably necessary for one action or another.  Do not delete them.  The program might still run, but not completely anymore. Some common assisting files are initiating (.INI), history (.HIS), high scores (.SCO), data files (.DAT), information (.INF) or configuration files (.CFG). The files need to stay together with the program (or stay in a defined folder).

A backup file has generally the extension .BAK.  When you change a file and save it, the old file is not always thrown away, but may be kept as a backup file.  If you change file1a.txt, the backup file will have the same name, but a different extension: file1a.bak. When you delete a file by accident, you may still have the old backup. 

A program often comes with an icon (.ICO). This is the little picture of the program that you can see in Windows. An icon is the image of the button to start this program.

Other pictures, that the programs use, can have the extension .BMP (Bitmap) or .JPG (Joint Photographic Experts Group) or .GIF (Graphic Interchange File). You can use these pictures yourself with a drawing program, or put them in a document as an illustration.  There are many different type of pictures, so your program might not be able to look at a picture of a different format.

The different letter types that you can use in some programs are called fonts. You can spot them in a list by their extension .FNT or .TTF (True type font)

The help that is available for a program is often stored in a file with the extension .HLP.

Music files are generally either .WAV (Wave) or .MID (Midi = Musical Instrument Digital Interface) or .MP3.  (Moving pictures expert group 3^rd level data compression) You can store music, or your voice!

Zipped files (.ZIP) are files in which the data is compressed. You cannot read the data unless you unzip the files first.