Multiple Snap-Ins (from multiple sources or vendors) can be assembled by an administrator into a tool (which could also be called a document). The tool is what the administrator actually uses to manage the network.
After assembling a tool from various Snap-Ins, the administrator can save the tool in a .MSC (Management Saved Console) file. The administrator could then reload the file later to instantly recreate the tool. The MSC file could also be emailed to another administrator, who could then load the file and use the resulting tool. (If the second administrator does not have all the necessary Snap-Ins installed on his or her computer, MMC will automatically download the needed Snap-Ins when the second administrator loads the .MSC file).
MMC permits total customization by the end-user; an administrator can construct the ideal tool from all available Snap-Ins. An administrator can create multiple tools, and load and unload them when needed.
You can run multiple tools simultaneously on one computer, but each tool requires its own instance of MMC to be running.
Note that with MMC, a single "tool" does not necessarily have only a single purpose (such as User Manager for Domains is used only for user and group management). It is likely that the one tool that an administrator creates and uses regularly will contain management functionality for all aspects of the network—the Directory, replication topology, file sharing, and so on. It is called a "tool" because it runs in one instance of MMC, and can be saved in one .MSC file.