VNC is a "remote control" program that allows you to control a computer over a network (either a local network, or the internet.) When set up properly, everything on the screen of the "server" computer is visible in a window on the "client," and the client's mouse and keyboard input are received by the server.
Both client and server software is available for OS X, and many other platforms as well. (If you have a PC, you can control it from your Mac over the network...very useful sometimes.) You will need a fairly fast network between your two Macs...dialup won't work, but a local ethernet or AirPort network will, and a broadband internet connection should.
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