Device Commands are custom defined functions, which you can define for every device type, like the right click tools in the sccm console. It's possible to run the commands on the server or on the client where the webinterface is opened. You can also use many variables for your commands.
The commands which run on the server are executed with the service account, the local commands with the user which is currently logged on on the computer. The commands which are executed on the client require some Internet Explorer Settings. See how to add a device command for more information.