Wide range of sizes, although none quite as small as halflings and none quite as large as goliaths. Skin and hair colors are most commonly along the brown and red spectrums (very light to very dark), although some humans go into the gray and bluish tones and this is not viewed as unusual. Eye colors are almost always blue and green, and sometimes gray, despite skin tone. Dark eyes are rare and are seen as a portent. Otherwise PHB rules.
All human races (including the ones below) are secular, and have no gods of their own. As such, divine-based classes are off-limits without a good roleplaying reason. To play a divine-based character, you would probably have to be an adherent of one of the elvish religions.
These are considered a normal human variant. PHB2 rules, except there’s no need to stick to the mountainous background – goliaths can come from anywhere – and goliaths have the same rarity of dark eyes that other humans have.
Orcs and humans have a common ancestor, but diverged around 600,000-700,000 years ago, long enough to have become completely incompatible. Half-orcs are not truly “half human, half orc.” They are a human population that maintained compatibility with orcs substantially longer than all other humans (another 1 or 2 hundred thousand years). Thus they held onto their orcish features.
They have no connection with orcish society, and half-orcs are never found among orcs. And although common usage will use the somewhat pejorative term “half-orc,” no one in polite society uses that label. They are just humans who are rather orc-y. It isn’t too common anymore to find someone with strongly orcish features, as the orcishness is being slowly bred out of the gene pool.
Note: Orcs are not Warhammer-style greenskins. Half-orcs don’t have green skin, either. Like all humans, they almost never have dark eyes.