The prototype property

OK,so can anyone show some uses of this property.
What does it return ?

I know that the props of all instances of an object type can be added/modified,like this:

myobj.prototype.newProp="..."


Again.what does obj.prototype return? because i've seen code that overwrites this property entierly...that is for user-defined objects,coz built-in objects won't let you do that...