MarcoPolo exists because the Mac OS X concept of a "location" is a collection of settings for your computer's networking devices. Switching your current location is much easier than changing all those individual settings, yet you still have to explicitly switch to the correct location each time.
MarcoPolo brings context-aware computing to your portable Mac computer. It allows your computer to determine its context through gathering evidence from your environment (evidence sources), using flexible rule-based fuzzy matching to make an educated guess (rules), and then performing arbitrary actions upon changing context (actions).
MarcoPolo's concept of contexts is a generalisation of a location, and encompasses more than just where your computer is. A context might represent what you are doing, or what else is going on around you.
MarcoPolo quietly stays in the status bar at the top of your screen (right-hand side of the menu bar), from where it can be configured to your own needs.
Note from the developer: "Sadly, I have not had time to maintain MarcoPolo for the last few years, and it is becoming less functional. For instance, the WiFi evidence source does not work on Mac OS X 10.6 or 10.7. Much of it still works, but you should consider this software unsupported."