At the high end there are a burgeoning number of companies with assistant products, like Siri and Alexa. At the low end, there are now hundreds of companies with DIY platforms for roll your own chatbot creation. These two ends are starting to meet in the middle, as the assistant products become more customizable, and the chatbot platforms become more specialized. There are a number of analyst companies, issuing breakdowns of this sector in various ways. Artificial intelligence, robotics, and the Internet of Things also figure into the equation, both off the high end and the low end.
See also my Quora answers to: