CSS: Top vs Margin-top

top is for tweak an element with use of position property.
margin-top is for measuring the external distance to the element, in relation to the previous one.

Also, top behavior can differ depending on the type of position, absolute, relative or fixed.

You'd use margin, if you wanted to move a (block) element away from other elements in the document flow. That means it'd push the following elements away / further down. Be aware that vertical margins of adjacent block elements collapse.

If you wanted the element to have no effect on the surrounding elements, you'd use positioning (abs., rel.) and the top, bottom, left and right settings.

With relative positioning, the element will still occupy its original space as when positioned statically. That's why nothing happens, if you just switch from static to relative position. From there, you may then shove it across the surrounding elements.

With absolute positioning, you completely remove the element from the (static) document flow, so it will free up the space it occupied. You may then position it freely - but relative to the next best non-statically positioned element wrapped around it. If there is none, it'll be anchored to the whole page.

Margin applies and extends / contracts the element's normal boundary but when you call top you are ignoring the element's regular position and floating it to a specific position.

Example:

html:

<div id="some_element">content</div>

css:

#some_element {margin-top: 50%}

Means the element will begin displaying html at the 50% height of its container (i.e. the div displaying the word "content" would be displayed at 50% height of its containing div or html node directly before div#some_element) but if you open your browser's inspector (f12 on Windows or cmd+alt+i on mac) and mouse over the element you will see it's boundaries highlighted and notice the element has been pushed down rather than re-positioned.

Top on the other hand:

#some_element {top: 50%}

Will actually reposition the element meaning it will still display at 50% of its container but it will reposition the element so its edge starts at 50% of its containing element. In other words, there will be a gap between the edges of the element and its container.

Cheers!