"The essential characteristics of the American Pit Bull Terrier are strength, confidence, and zest for life. This breed is eager to please and brimming over with enthusiasm. APBTs make excellent family companions and have always been noted for their love of children. Because most APBTs exhibit some level of dog aggression and because of its powerful physique, the APBT requires an owner who will carefully socialize and obedience train the dog. The breed’s natural agility makes it one of the most capable canine climbers so good fencing is a must for this breed. The APBT is not the best choice for a guard dog since they are extremely friendly, even with strangers. Aggressive behavior toward humans is uncharacteristic of the breed and highly undesirable. This breed does very well in performance events because of its high level of intelligence and its willingness to work." ...the UKC
The reality of dealing with dogs from shelters is that none of us knows a dog’s lineage.  Most often we find ourselves making a guess about the breed or breeds represented in a particular dog.  And let’s face it, even if we could be certain of a dog’s breed that alone doesn’t allow us to predict behavior. So in choosing dogs for our program, we focus on individuals that exhibit physical characteristics and most importantly personality traits that are in keeping with the standards for the APBT.

To familiarize yourself with these standards, please read this from…
  the UKC.
From BADRAP.org... Immigrants brought their dogs across the ocean along with their families and prized possessions. They soon became a fixture in a developing nation. In early America, the dogs were valued for much more than their fighting abilities. They were entrusted to protect homesteads from predators and worked as vital helpers on family farms. Homesteaders depended on their abilities to help in hunts and as hog catchers (hence, the common title "catch dogs"). They were constant companions to the young children who were entrusted in their care. Pit Bulls earned their place as an important part of the fabric of a developing nation.

As cities sprung up, Pit Bulls remained a prominent part of the American culture. The USA admired this breed for qualities that it likened in itself; friendly, brave, hardworking, worthy of respect. Pit Bulls were thought of less as pit fighters and more as 'regular dogs'. They show up in hundreds of turn of the century photos, flanked by loving family members. Early advertisements, posters, and magazines began to use the image of the All American Dog, including Buster Brown, whose companion was a Pit Bull.

World War I posters displayed illustrations of APBTs as proud mascots of neutrality and bravery. Not to disappoint, the most decorated war dog of that time was none other than 'Stubby' -- a loyal and brave defender of America's freedom. more at BADRAP.org
The American Pit Bull Terrier is a storied breed. And while we believe knowing where you came from is important, we will not reinvent the wheel by reiterating historical information here. We encourage you to do some research and because there has been so much written about the breed, we recommend these two sites as starting points...
From realpitbull.com..."Much of dog history is speculation, and quite akin to the piecing together of puzzles. Because of this, opinions vary about exact details of breed origin. We are fortunate in that the American Pit Bull Terrier, and its chief ancestor the bulldog, have a fairly well-documented history. Even so, debate occurs when trying to establish something as simple as whether or not the Pit Bull is the original bulldog, or whether it is, as popular short-histories insist, a 50/50 cross between the brachycephalic bulldog of England (the ancestor of the modern day AKC Bulldog) and now-extinct hunting terriers.

Part of the reason for the  confusion lies in the fact that until very recently, many dogs were classified and named according to general appearance and job function, not so much by "breed". Historically, the words "terrier" and "bulldog" were used quite frequently, but had ambiguous meanings. This makes it  especially difficult to trace the Pit Bull's exact ancestry.

Bulldogs and terriers are mentioned in the breed's history, but WHICH bulldogs and  terriers should we be considering? Presented here is a well-researched document on the history of the breed, along with bibliography to enable easy research for the interested reader. The reader is encouraged to further study the history of this most fascinating breed, for in its history lies the essence of the animal--an understanding of its history will give one an understanding of the breed...." more at realpitbull.com