Wow, I'm actually delivering on a pledged post within a week. OK, here we go, into the wonderful exciting world of synchronization of electric generators!
Maybe you don't know it, but that juice your electric utility provides you changes direction 60 times a second (assuming you're in the US - Europeans do it slightly differently of course). That's why they call it "alternating current", or AC. There's a diagram on this page with some other stuff too.
Generating electricity that way seems like a lot of trouble, but actually it's easier than producing nonreversing "direct current" (DC). DC requires rectifiers and other impedimenta, and was economically impractical at the time when our electrical distribution network was being built out.
But that alternation creates some problems. For one, it affects the impedance of electrical appliances - vary the rate of alternation and all of a sudden your motorized appliances might overheat. For another, it requires all generators hooked up to feed the system to be exactly in step with the existing rate and amplitude of alternation. That's where synchronization comes in.
They might have better metering stuff nowadays, but many plants synchronize with something called a synchroscope that looks something like a clock with only one hand. The hand rotates at a rate proportional to the difference in speed between the generator being synchronized and the grid it's being synchronized with. The hand position indicates the relative phases of the waves - when it's at 12:00 the waves are exactly in step.
So when you start a prime mover like a diesel generator or a steam turbine, you manipulate its speed with its governor. You set the speed up just a hair higher than that of the grid (as indicated by a slowly moving hand on the synchroscope), let the synchroscope's hand sneak up just short of 12:00, and then you close the breaker to connect it to the grid.
Whoops, you weren't in sync when you closed the breaker? Well, lots of neat stuff can happen then - just don't stand too close. You might fry the windings on your generator. You might jerk your prime mover loose from its mountings, or tear something loose internally. If your generator is big enough relative to the power distribution grid you're supposed to be feeding, you could even bring the whole grid down. That's not a good career move. (And unless you think it's a good idea to redline your car's motor in neutral and then slam it into gear, don't try starting a diesel generator by just connecting it to the grid. That was done at a plant in California back in the 80's, and they managed to break the ~13" crankshaft on a diesel generator).
Well, there's more to it than that yet. You also have to fool with the voltage regulator. This varies current in the exciter to get your generator's voltage at the appropriate value. You want the power to flow out, not in, you want the right power factor (no, not another pledged post!) and you'd better have some reverse power relays ready to knock you offline to protect against idiots and emergencies.
Nowadays more and more of this process is automated. But these functions above still must be carried out in some form or another. And with any luck, if you ever use a backup generator, I hope I've convinced you to read the instructions.