Just about every Mac ever built is network-ready. Buy a couple of components, and you can connect all the computers in your office together, with or without wires. The process of rigging the hardware and software for a network is somewhat more technical than, say, emptying the Trash, but it's not exactly rocket science.
Once you've got a network, you can copy files from one machine to another —even between Windows PCs and Macs—just as you'd drag files between folders on your own Mac. You can send little messages to each other's screens. Everyone on the network can consult the same database or calendar. You can play games over the network. You can share a single printer, cable modem, or fax modem among all the Macs in the office. You can connect to this network from wherever you are in the world, using the Internet as the world's longest extension cord back to your office.
And in Mac OS X 10.5, you can even do screen sharing, which means that you, the wise computer whiz, can see what's on the screen of your pathetic, floundering relative or buddy elsewhere on the network. You can even seize control of the other Mac's mouse and keyboard. You can troubleshoot, fiddle with settings, and so on. It's the next best thing to being there —and often a lot better than being there.
You don't necessarily need a network to exchange files between Macs (and even PCs). If each computer is equipped with Bluetooth circuitry, they can shoot files ...