The flight from A to B might be a very busy route, with several airlines serving that market, while the A-C and C-B routes might be not as busy. Usually such hidden city fares occur when A and C aren't hubs for the carrier in question, but B is, and B is dominated by the airline. So the airline routes you through B, but charges you less than if you were to purchase two round-trip tickets.. 

Theoretically, you could buy a ticket from A to B, get off at point C, and throw away the B to C portion, saving some money, if point C was your ultimate destination. Airlines frown on this practice, and taking advantage of a hidden city fare is explicitly forbidden by their rules. If you happen to skip a leg of your flight (e.g., logged as a no-show on the airline's computer), the airline has the right to cancel all subsequent legs, and will do so to discourage folks from using hidden-city fares. So the only case in which you can "safely" take advantage of a hidden-city fare is when you're taking a one-way flight. If you buy a round-trip ticket from A to B through C, skip the C to B leg, and try to board the return flight at B or C, you'll find that your reservation has been cancelled and you'll be required to buy a new ticket at the full-price one-way fare. 

You also can't take advantage of a hidden city fare if you've checked any baggage, as your baggage will be sent through to your ultimate destination. 

Airlines have started to really crack down on the use of hidden city fares. They can not only cancel subsequent flights on their own lines, but also recommend cancellation of subsequent flights on other carriers. They've programmed their airline reservation systems to watch out for hidden-city reservations, flagging potential violations in the passenger's record, and in some cases will automatically cancel all subsequent legs if one leg is skipped. Even if the reservation system doesn't automatically cancel the subsequent legs, the agent at check-in will see the warning flag and will be very suspicious of any skipped legs. 

Some airlines (e.g., Delta) have a practice of checking you in for all outbound flights at the point of origin. But this doesn't make them any more susceptible to folks who use hidden city fares, because if you don't show up for the return flight at B, they'll still cancel all your remaining legs. Also, sometimes the boarding passes are marked "check-in required". 

Hidden city fares happen most often when the connecting point is dominated by one airline (the carrier of your flight) and the ultimate destination is a competitive market.

If you happen to catch an earlier flight than your scheduled one, be sure to reconfirm your subsequent flight segments. Any departure from your ticketed reservation can potentially cause your itinerary to be flagged as a hidden-city violation (e.g., "NOSH" for no-show), if the gate agent didn't record the earlier flight properly. 

If you're the dishonest type and are going to lie about actually having taken the skipped segments, at least have the intelligence to remove the ticket and boarding pass (keeping the stub of the boarding pass) from the ticket packet. [I once saw a couple try this stunt in New York, and the gate agent caught them at it. The wife had removed her ticket and boarding pass; the husband hadn't. The husband claimed that the gate agent at the hidden city had forgotten to remove the ticket. The gate agent didn't let them on the flight because the computer showed that they had missed TWO segments of their flight -- from the hidden city to their ultimate destination and back. In addition, the gate agent had been on duty the last time they passed through, and didn't remember seeing them board. The agent's supervisor concurred.] 

If you know in advance that you want to skip a segment of your flight (e.g., you're flying from A to C via B, but want to get off in C, visit with some friends, then drive up to B to visit some more friends and return home), tell this to the travel agent when you buy the ticket. They can make a note about it in the record so that your return flight won't be automatically cancelled when you miss the B to C leg.