Alexa Amazon is a hands-free speaker you control with your voice. Echo connects to the Alexa Voice service to play music, make calls, send and receive messages.
Beyond asking for music, you can use Alexa to search Wikipedia (“Alexa, who was Steve Jobs?”), make quick cooking conversions (“Alexa, how many milligrams in a kilogram?”), or create a to-do list (“Alexa, add ‘book a tennis court to my to-do list.”) A growing list of built-in capabilities and third-party skills means that your Alexa device keeps improving the longer you own it. Alexa services being offered free to third-party gadget and app developers. It's an exciting concept because it means more people benefit from tech advances. The more the bots can do, the more the novices benefit. And, that leads to another exciting prospect. Someday, Alexa and other bots will replace your phone. We won't have to learn new apps or bother trying to configure all of the settings.

Google assistant is Googles voice-controlled smart assistant originally upgraded of
Google Now. Google Now works from within the google android and iOS devices, though
its hardware-controlling capabilities are limited on iOS. The app lets you quickly search the web and perform a variety of tasks such as set alarms, adjust volume and post to social media using natural voice commands. Like any other virtual assistant Google Now offers hands-free accessibility from the lock screen with simple voice command: ‘Ok Google’.

Siri has been introduced back in October 2011 when Apple released iPhone 4S. Back
then talking to a device to set alarms or answer messages was seen as
revolutionary. However, in 2018 Siri have fallen behind other digital
assistants. The apple company typically waits until new features or technology
are fully polished, in contrast to Google’s approach of launching beta services
and iterating in public. Siri is designed as a human language personal
assistant.

Nearly every smartphone and computer on the market today has smart assistant built in.
Cortana is designed to recognize context, so it should be able to understand
follow-up requests.
Cortana is an operating system-oriented voice assistant available on Windows 10
devices, as well as the Xbox One console. Cortana can help answer general questions by pulling information from Bing. Cortana is designed to recognize context, so it should be able to understand follow-up requests. As a virtual assistant it constantly analysing your interactions to learn more about you, it can read your emails track your location, watch your browsing history and much more. While Cortana can answer basic questions, there a multitude of scenarios in which Cortana or any other competitive virtual assistant is practically incapable of conversation.

Bixby is an artificial intelligence system that is designed to make device interaction easier. It works across a range of Samsung products including smart fridges, TV’s and more. Samsung’s high-end Android phones, come with their own voice assistant called Bixby.
The service essentially works in the same way as other Virtual Assistant solutions like Google Assistant or Vivi it listens to your voice, interprets the information and returns the resulting action. The contextual awareness means you can get it to take actions without laboriously detailing exactly what to do with what - it already understands where you are so can take the logical next step, so there's no need to use the precise language that you
did on older voice-control systems.

The digital assistant provides Messenger users with in-chat suggestions and services.
Unlike some digital assistants, M is not voice interactive but pops up in users’ text chats.
Facebook M is the social media network’s artificially intelligent assistant that can offer suggestions in your conversations. M itself is powered by Facebook-built AI, it works by opening up in your chats when it detects that it can help with something.

The digital assistant provides Messenger users with in-chat suggestions and services.
Unlike some digital assistants, M is not voice interactive but pops up in users’ text chats.
Facebook M is the social media network’s artificially intelligent assistant that can offer suggestions in your conversations. M itself is powered by Facebook-built AI, it works by opening up in your chats when it detects that it can help with something.