Drop is a library built on the Tether positioning engine. Tether is intentionally designed to only handle positioning. You give it an element and a target, and it keeps the element where you want it relative to the target.
Quickly it became clear that while that simplicity allowed Tether to be more powerful, many of its use cases required a few more concepts. Specifically, Drop adds the following on top of Tether:
It's very possible that a sufficiently complex application will eventually outgrow
Drop and move to using Tether directly. You can always pass
to your Drop instance if you need more explicit control of Tether's behavior.