A file or document always has a file type, which tells how the information is stored in the file. A program can only work with certain file types. The file extension (the letters after the dot in the file name) are specific for a file type. The extension for PowerPoint presentations is ppt. (Your computer may not be set to display the extensions.)
A specific sort of file, such as a text file or a graphics file. File types tend to have different file extensions - .txt stands for a text file. As many files are not just text files, they need to be associated with an application which displays them properly. In Internet Explorer, a file type screen is displayed to associate file extensions with file types and applications. The registered file types are automatically used by Internet Explorer - the user does not need to redefine them.
Programs each label files with a code to identify which program created them. That is how they attach the right program to a file when you open it. Some files can be opened by programs other than their creators, but that ability has to be built into the program. Graphics get complex in this category. PCs do this by attaching a File Extension to the end of the name - .exe, .txt, and so forth. Macs do this by invisibly coding it with a 4 letter code.