Humans (Homo sapiens) are one of the most common species of sentient and sapient life in the multiverse.
Most humans, unlike Nobby Nobbs of the Discworld, do not require a paper showing that they are indeed human beings to be recognized as such. They tend to come in a variety of skin tones, ranging from very dark brown to very light pink, and have two arms and two legs attached to a main torso to which their head is also attached. Once out of infancy, humans stand upright on their legs. They have four fingers and a thumb on each hand, enabling fine manipulation of objects. The front of the head contains a mouth, a nose, and a pair of eyes, enabling binocular vision. Most humans have hair growing from the top and down the back of their heads. Humans do not taste like chicken. In fact, being omnivores, humans taste closer to pork.
Humans are often the 'baseline' sapient species that a continuum uses to measure all other sapient species/'races' against. When found alongside other sapient species, their most defining traits on whole tend to be adaptability, a love of order, uncommon determination, and their outgoing nature in the face of strife. However, as seen in the Real World, a more realistic view of humans is that they are varied and can possess all sorts of personality traits.
In continua that do not have humans, there may be a species that is near enough to Homo sapiens as we know it that humans can understand and sympathize with the characters.
Most (if not all) writers are human, including fangirls and fanboys, so capturing the mindset and behavior of other sapient species can sometimes prove difficult for them without sufficient experience.
Many Sues claim to be human, but it is unclear if they actually are human (or any known species!) or not. Considering that they often ignore human biology, human mentality, human experiences, and human capabilities, they certainly do not seem to be very human at all.
Humans and the PPC
Due to their numerous nature in the multiverse, many PPC agents are human, but may vary in appearance, abilities, and/or culture depending on where in the multiverse they hail from. A human from Azeroth might not understand the same things as a human from Earth, or a human from Arda, or a human from the Discworld, or a human from Thedas. Humans from the same continuum might not even understand each other if they come from different time periods or vastly different cultures.
Although PPC HQ is nearly unnavigable, many parts of it seem to be designed with humans in mind. This makes sense considering that Makes-Things, a human, was the first non-Flower member of the PPC. Thus, at the time HQ was constructed, humans were likely one of the only species considered. This may prove inconvenient for PPC staff that are larger or smaller than humans, or those that possess vastly different anatomy. It seems that most doorways, furniture, bathrooms, dining options, and other features are provided with humans in mind. For non-human agents, this may prove to be inconvenient, hilarious, or both.