Biology and Psychology
Humans employ a typical primate physiology of two forelimbs called arms with grasping appendages known as hands (each with a usual count of five distinguishable fingers) and two hind limbs recognized as legs with flat-bottomed (or very slightly curved) appendages called feet. Human legs tend to be longer than arms and provide locomotion for humans as they walk upright (or at least relatively so) while the arms and hands are used for manipulation of tools.
Adult humans stand at an average of five and one-half feet tall, but the range of human height varies significantly, with maximums reported at nearly eight feet tall in extreme cases and three feet or less in equally rare events. Weight is typically but not always correlated with height, and muscle mass as well as fat content can determine the weight of a human.
Humans come in a variety of colors, which here means a tongue-in-cheek way of saying "50 Shades of Brown". Pigment in human skin is determined by melanin content which in turn is a reaction to climate over long periods of time. In some universes and on some planets, pigment content is a source of contention or social discrimination, usually a case of a group of individuals with a particular pigment being in power and using that power to devalue the morality of or prevent resources to a group with a different pigment. This is typically considered by civilized individuals to be a horrible endeavor and should be prevented at all cost.
Humans tend to view themselves as a "baseline" species when they are the only sapient creatures they are aware of. All other creatures are then considered, to them, as some level of "near-human", "non-human", or (in the case of marked similarity in anatomy) "humanoid". These are useful (though biased) measures of comparison which tend to be discarded after peaceful communication with other sapient species leads to multiculturalism and better understanding of their species' place in the universe.
Unfortunately, peaceful communication tends to be long in coming or nigh-impossible as humans are known to be a particularly war-like species. Constant competition for resources on their own planets and failed consistent unification of governments leads to fractured alliances and nearly constant warfare. Smaller alliances and individual partnerships are common, but religious differences and differing social customs between human nations tend to cause rifts between most nations which cannot be repaired easily. Hope is often held out and indeed, an infinite number of universes exist in the Omniverse where human unification is achieved, but it is most often found through extraordinary circumstances.
Humans in the Omniverse
The Omniverse in its infinite nature contains an infinite number of multiverses in which humans exist and are the central characters of the stories which govern their respective universe. Similarly, there are an equally infinite number of multiverses in which humans never evolved or developed whatsoever.
Unfortunately, being humans as we are, the creators of this wiki tend to focus on those multiverses/universes/timelines in which humans take center stage under the "Write What You Know" principle. Nevertheless, it is important to remember that on an Omniversal scale, humanity is only one in a long list of sentient races which are equally insignificant next to the scope of Infinity.
Humans and Earth
In many universes (including Real Life), Earth is the home planet of the human race. Many of these universes include having humans as the dominant species of life on the planet, meaning that humans are the only ones to have evolved to the point of creating societies, language, government, and control of resources on a grand scale. The humans of Earth are a volatile race, capable of great feats of technology and great lapses of moral consciousness in equal measure.
Humanoids are "near-human" races which share the same basic tertiary physiology as humans. Humanoids are characterized by having the same external body structure; that is, humanoids tend to have an upright gait with lower appendages built strictly for locomotion and upper appendages used for the manipulation of objects. A head structure which contains the majority of information storage, hormone secretion, and primary sensory information tends to be common as well.
The definition of a "humanoid" varies within the setting. The most simple test would be to compare the silhouette of whatever species you are discerning with that of an average human, to scale. If at least 75% of the silhouette matches, you are probably dealing with a humanoid, if not merely a non-average shaped human. This might account for something like an extra set of appendages, but not for multiple sets or for a completely different body shape.