Humans are one of the Alpha Quadrant's dominant races, one of the three most common along with the Klingons and the Romulans. Humans come in two sexes, male and female. The male is the taller and larger of the two, averaging 175 centimeters in height and a mass of 78 kilograms. Females, smaller, average 162 centimeters and 62 kilograms. There is obviously considerable variance around these median values, but this has grown smaller and smaller over the last several centuries. This 'regression towards the mean' is a natural consequence of evolution.
Skin color ranges from black along shades of pink to a very pale peach, depending on the person. Women typically have slightly lighter skin than men. Humans usually have naturally occuring hair colours of red, brown, black, or blonde, with grey and white appearing as a person ages. However, the availability and safety of hair dyes in the 23rd century mean that hair can literally be any colour detectable by the human eye (although some organizations do not smile upon hair dye in their members). In the 23rd and 24th centuries, rising human life expectancies have meant that a healthy human has a fairly good chance of reaching one hundred years old over the course of their lifetime.
Humans are the dominant lifeform of the United Federation of Planets, and almost every Starfleet Commander-in-Chief and President of the United Federation of Planets has been a human. Only two Fleet Admirals, Clayton Stabu and Jacien, were non-Human, and they were both Vulcans. Humans are native to the Federation capital of Earth, although they have ranged out to several planets including Centaurus, Remulak, and the other colonized worlds in the Federation. Only a handful of humans have ever lived outside the bounds of the Federation, and defections to the Klingon and Romulan Empires are so rare as to be unspeakable.
In terms of physical and mental traits, Humans are the average by which all of the other races are defined. Literally the average. There is some variance in all of these traits but, naturally, these vary from person to person.