Basically, a 'machine' that uses many ordinary items, and positions them in a way so that when activated, sometimes by 'knocking down the first domino' so to speak, it leads to a series of events that are all activated by the previous events.
They are most always elaborate, with many things going on at one time.
They are hard to build, and sometimes take over 4 months and a millions hours to build, but the results are spectacular.
Child 1: Hey, let's play Mouse Trap!
Child 2: I heard that's something called a Rube Goldberg Machine!
Teen 1: Have you seen Ok Go's new video, 'This Too Shall Pass'? It's totally a Rube Goldberg Machine!