Certain files on your computer are associated with a corresponding program capable of opening it by default. When you double-click on the file, the associated program opens it or performs the default action on it. For example, a .txt file would be opened by a text editor application such as Notepad. These associations can be changed so that a different program opens the file by default.
A file extension is the suffix at the end of a file name. For example, for a file called: "FileName.txt", the suffix "txt" is the file extension. This information is used by the computer for the purpose of identifying the format of the file and the software that is associated with it. In this example the file is a text document. While most extensions are three characters in length, they can be one to five characters long.
In situations where an application is uninstalled there can be associated files that remain on your system. When there is no associated software capable of handling them, orphaned files cannot be opened.
A version of the software application that enables you to try out the product before registering to buy it. After the trial period has ended you can continue using the product for free but with limited functionality, or you can get a license key and activate the product to receive full functionality of the program. For more information, see How to Register.