Below is a quick summary of the differing Amazon Alexa devices
Amazon Echo - The main feature that sets the Echo apart from the rest is the speaker that it has built-in. The speaker has hit criticism from some but the ability to play music with a good sound without relying on an external speaker speaks for itself.
Amazon Tap - The Amazon Tap is the only mobile solution. It comes with a charging cradle and once charged the Tap can run for around 9 hours. This makes it perfect for a day out. Of course, unless you are streaming audio from your phone you would need a wi-fi connection to connect it too. 1 downside of the Tap is that at present it is US only.
Amazon Dot - The Amazon Dot is much smaller than the other 2 and is ideal for if you want a smaller discreet device or if you would prefer to hook up to your own speaker via the 3.5 mm jack. If you wish to use Bluetooth do watch out however as the Dot does not work with speakers that require a pin.