What is the difference between Echo, Tap and Dot? It is confusing me a bit. Dot: has no speakers? Requires bluetooth based pairing. Requires an Echo to work?

What is the difference between Echo, Tap and Dot? It is confusing me a bit. Dot: has no speakers? Requires bluetooth based pairing. Requires an Echo to work? Tap: has wireless speakers with a built in battery. Also seems to have a Mic. Do I need an Echo to make this work? Can the tap work with the dot?