The term "human nature" refers to the natural, instinctual drive that is within most humans (there will always be exceptions to the rule, and they stick out like sore thumbs). For example, it is basic human nature to want to protect your children and make their lives better than yours has been. It is basic human nature to grieve loss, celebrate success, and become filled with ennui when life becomes a bit too stolid and stodgy. It is human nature to crave novelty, and become bored with the same-old same-old.
There are particular actions, hopes, and desires that are ingrained in humans, not matter what their upbringing, training, or education taught them. For example, the instinct to survive is very strong, so when threatened, humans will struggle frantically against the danger. It is also natural (human nature) to want to impress the opposite sex, it's a matter of reproduction and survival of the human race. Another way to phrase it is to say human nature = what comes naturally to humans.
I can't add much to what PVixen and Koichan said. They pretty much covered it. Although I will say that I think one way to see an example of the basic form of human nature is to watch a 3 or 4 year old. They often exhibit most aspects of what we call human nature, good and bad, in mild baby form. I think a lot of our cultural and societal rules were created to rein in some of the more destructive aspects of human nature and reward the advantageous ones. Just MHO. :)
In The African Queen, Charlie, a drunken boat captain, attributes his penchant for gin to human nature. "A man takes a drop too much once in a while, it's only human nature," he says. Rose, an Anglican missionary, responds, "Nature, Mr. Allnut, is what we are put in this world to rise above." Charlie knows, experientially, that "human nature" is inherently weak. The unbending Rose refuses to accept natural weakness as an excuse for sin.http://www.compellingtruth.org/human-nature.html