A snap-in is the basic component of an MMC console. Snap-ins always reside in a console; they do not run by themselves.
When you install a component that has a snap-in associated with it, the snap-in is available to anyone creating a console on that computer (unless restricted by a user policy). For information about user policies, see Group Policy and MMC
Stand-alone and extension snap-ins
MMC supports two types of snap-ins: stand-alone snap-ins and extension snap-ins. You can add a stand-alone snap-in, usually called a snap-in, to a console tree without adding another item first. An extension snap-in, usually called an extension, is always added to a stand-alone or extension snap-in that is already on the console tree. When extensions are enabled for a snap-in, they operate on the objects controlled by the snap-in, such as a computer, printer, modem, or other device.
When you add a snap-in or extension to a console, it may appear as a new item in the console tree, or it may add context menu items, additional toolbars, additional property pages, or wizards to a snap-in already installed in the console.
Adding snap-ins to a console
You can add a single snap-in or multiple snap-ins and other items to a console. In addition, you can add multiple instances of a particular snap-in to the same console to administer different computers or to repair a damaged console. Each time you add a new instance of a snap-in to a console, any variables for the snap-in are set at default values until you configure the snap-in. For instance, if you configure a particular snap-in to manage a remote computer and then add a second instance of the snap-in, the second instance will not automatically be configured to manage the remote computer.
In general, you can only add snap-ins that are installed on the computer you are using to author a console. However, if your computer is part of a domain, you can use MMC to download any snap-ins that are not locally installed, but that are available in the Active Directory directory service. For more information about adding published snap-ins and extensions to a console, see Creating consoles For more information about distributing software by using Active Directory, see Help.