I'll give a quick runthrough
position:relative (this is most often used to provide a parent (a starting place) for Absolutely positioned elements. Aka if you have this
And you set coordinates for the <absolute>...it would base the starting point at the boundaries of <relative>. Otherwise, it would normally start at the viewport. Positoin:relative (combined with coordinates) basically just creates an imaginary box over the element, with all the colors/what not. And if you move it with coordaintes, you're moving that imaginary box with the colors. Thus the original element stays in place. It still retains where it is in the documents flow
Position:absolute; removes elements from the flow completely. Without coordinates, the browser guesses where to place it. With, it will go to the nearest parent with position:relative (or any other position value) and use that as a reference. Generally you want to avoid using this in most pages. At least for structure anyway
Position:fixed, basically the same as absolute, however, it will stay on the screen (assuming you set coordinates that make it on screen). Even with scrolling, it will stick on the viewport. No getting rid of it :).