Dogs are a lovely species of animal, but for as lovely as they can be, they can also be just as wild and out of control at times. There are often a lot of things that go into why the dog progresses to that point, but most often, it is a problem of learned behavior from owners or of bad behavior being allowed to become worse through inaction. At that point, it may become all too much work to reign in your pup, and you may be advised to consider taking your dog to a dog obedience school. There are a lot of questions you may have however, such as effectiveness, treatment, cost, distance, etc. that will all be answered shortly.
Your mileage may vary, but depending on the quality that you expect, it can be a little or it can be a lot. While there are exceptions, your life should never be defined by getting the best of the best for the least. It just does not work that way, unfortunately, and quality dog training does indeed come at a bit of a hefty price point.
Abuse at these facilities is not as much a concern as it once was, but that does not mean that the concerns about abuse are invalid. You should make sure that you go to an accredited facility and to read reviews; if a non-trivial percentage of people online are talking about poor facilities, treatment, or trainers, back out!
The common behavioral modification practices at dog obedience schools involve teaching verbal commands and instilling good values. Dog obedience schools will not use physical punishment as it is considered abusive; instead, good behavior is rewarded with treats, thereby conditioning the dog into realizing that the good action has a positive outcome for them, and the bad action lacks any such thing.
This will ultimately depend on the dog. Sometimes, some dogs are simply beyond help and will need extensive training to iron out all of their problematic behaviors, while other dogs will take to their training like a fish to water. The quality of the trainer and your enthusiasm will also have an effect.
This answer is all rather variable; in general, it is good for a dog owner to be present during the training, as if they are not their dog may receive training exclusively from a stranger, and when you are given the reigns, they may not register that they should respond to anything you say or do, as you were not the one training them.
Quite effective, though this is of course a standard answer for any product or service; the less you pay, or the lower reputation of a company, the less you can expect in the end. Dog owners who bring their pooches to a dog obedience school often find generally positive results. Perhaps not perfect, but a noted improvement and one that is worth the cost.
That all depends on the school as well as you. If your dog has relatively minor problems, it is not all too uncommon that they may just have you bring your dog less often than, say, a dog that straight up will not listen to its own name or acts ornery around the house.
Dog obedience schools with a decent pedigree (no pun intended) usually have dealt with a variety of problems; aggression, laziness, "disrespect," etc. So in general, a dog owner is likely going to enjoy the gamut of training options. And while it may seem like aggression would be the hardest to deal with, they're all difficult traits in their own ways.