American Staffordshire Terriers (aka Amstaffs) are a lot like American Pit Bull Terriers (APBT). They are both intelligen, stubborn, sweet animals with humans, but you have to watch out for aggression with other animals. You should have a game plan in place before you bring one home about how everyone in your family will treat the dog, how it will be trained, and have your first trainer lined up.
They are NOT the hardest dogs to own - BUT - if this is your first dog as an adult (meaning you had dogs as a kid, or this is your first dog ever) then you will be in for a very bumpy ride. This is not a beginner's breed.
Training using aggression on your part, instead of positive reinforcement, will not be suitable for this breed. Positive reinforcement works wonders even when they are in stubborn mode. Jerking, spanking, rolling, or trying to dominate the dog via force will lead to problems... as the dog may decide you aren't worth submitting to. Forcing a dog to fear you does not mean it will trust you.
Instead positive reinforcement mixed with NILIF, using treats, praise and firm rules, will get you to your goal much faster, as well as teach your dog to trust you.
Physically they are small stocky dogs (17-19" at the shoulders). They are about 50lbs or less. They come in many different colors and patterns. Unlike pit bulls they do not have red noses. The noses are always black in pigment.
They can suffer from hip dysplasia, temperment problems, eye problems, and heart problems. This is especially true of poorly bred Amstaffs.
