1. What is a pit bull?
"Pit bull" is an abbreviated form of American Pit Bull Terrier (APBT), and nothing more. The APBT is a strong, compact dog that was originally bred from the ancient bull dog (quite different from a modern bull dog) for bull baiting. After that sport was banned, they were used to fight each other. More responsible uses of this working dog have included hunting, security, athletics, and therapy.
2. Isn't it an umbrella term?
One horribly common but inaccurate use of the term is as a catch-all for any dog with a muscular frame and a blocky skull. This has included everything from the preciously small "pocket pittie" to the monstrosity know as The Hulk. Breeds commonly lumped into this category often include the American Staffordshire Terrier*, American Bully, Bull Terrier, etc. Unfortunately, many shelter dogs also pick up the label if they fit the profile. This is almost always a death sentence. Please do no use pit bull as an umbrella term. Instead, the term "bull breed" is more appropriate and causes fewer problems.
* Depending on which kennel club you ask (AKC or UKC), American Pit Bull Terriers and American Staffordshire Terriers may be considered the same breed.
3. Why does it matter?
Here's why this is so important: when we lump together distinct breeds into one large category, we also lump together their bite statistics. Let's look at an example:
In a completely imaginary city, with completely imaginary statistics, grouping all of the "pit bulls" together suggests that those dogs bite at a much higher rate then other dogs. If those "pits" were to be accurately separated into their actual breeds, we see that they bite no more frequently than other dogs. As one might imagine, combining the statistics of "hound" or "retrievers" might cause a similar incorrect analysis.
Poorly kept records such as these have resulted in pit bull bans in many cities (or, in some cases, entire nations). Montreal is the most recent one to make headlines and create an uproar in the dog community. In other areas, "pit bulls" may not be in public without a muzzle and must be be kept on a short leash.
4. So what's Grimm?
Since Grimm came from an un-papered litter and his mom didn't care enough to ask for any information about his parents, it's impossible to be sure. His physical appearance suggest that he is at least mostly APBT. His size, however, indicates that something larger got mixed in somewhere. True pit bulls shouldn't reach 75 lbs.
Doggie DNA tests have been shown to occasionally be imperfect, but Grimm's mom is considering ordering him anyway. It's as close to the truth as she could possibly get, and it would satiate at least some curiosity. The timing of Zak George's review of the Embark DNA Test (and coupon code) seems to be just too perfect to ignore.