Tooltip displayed through long press

A tooltip is an element containing simple text content describing a particular element. It's hidden until the user desires more information from the element, e.g. before deciding to click a button.

A popover is an interactive HTML tooltip. It can be a dropdown, menu, or any other kind of box that pops out from the normal flow of the document. This type of element contains non-vital functionality and can be hidden behind a click or hover to conserve space.

Both are elements positioned near a "reference" element, and are hidden until they are triggered. They help conserve space by hiding secondary information or functionality behind a hover or click. They are positioned outside the normal flow of the document so when they are triggered, they are overlaid on top of the existing UI without disrupting the flow of content.

Popper has a sole goal: position elements. That's why we call it a "positioning engine" and not a "tooltip library".