Amazon Echo Dot
The Echo Dot works just like the Amazon Echo except that it's smaller, cheaper and connects to your existing audio system instead of relying on a large built in speaker. This means it can be placed much more discreetly around your home and take advantage of your existing higher-quality audio set up. The Echo is still a better all-in-one solution, but the Echo Dot is less expensive and a better option if you don't need the built in speaker or want to expand the range of your existing Echo around the house.
Note that you don't need the Amazon Echo to use the Echo Dot.