The Hero lives in his ordinary world. Something happens that is a "call to adventure" – something is needed or is lost or must be done. The Hero resists, but very soon has to set forth on his long (or metaphorical) journey.
Once on his journey there are obstacles; people are there to help and hinder him; until eventually, after a big struggle with his Nemesis (opponent), the Hero gets what he was after (usually).
Now he must return to where he started from. Again there are problems and perhaps reluctance. During the return there will usually be one last huge struggle in which the Hero will face death before finally defeating the Nemesis. He returns to his "ordinary world" a changed man, and the thing he brings back changes the world too.
 I will be referring to the Hero as "he" most of the time even though a Hero can obviously be female or be an object or a species with no gender.