Each MMC tool is built of a collection of instances of smaller tools called MMC Snap-Ins. One Snap-In represents one unit of management behavior. A Snap-In is the smallest unit of console extension. Technically, a Snap-In is an OLE In-proc server that executes in the process context of MMC. The Snap-In may call on other supporting controls and DLLs to accomplish its task.
Snap-Ins extend MMC by adding and enabling management behavior. This behavior may be added in a number of ways; for example, a Snap-In might add elements to the viewable node namespace (Microsoft's Directory Service Management Snap-In will enable the network Directory to be viewed in MMC), or it might simply extend a tool by adding context menu items, toolbars, property pages, wizards, or Help to an existing Snap-In.