Bots are computers. In particular, they are tasks or features that a computer can do rather than a person. Messaging bots do things like answer common questions, tell jokes, or look up information such as the weather or account data.
Messaging or chatbots can use any number of channels: SMS/MMS (normal text messaging on your mobile phone), Facebook Messenger, Twitter, and many other text-based apps and services. One example I like is Aeroméxico's WhatsApp bot. It is still in pre-release and not available publicly. It's reported features will include searching for new flights, checking flight status, booking flights, and getting notifications about flights. It means that customers can use WhatsApp as their whole interface to Aeroméxico. That's powerful and cool.
Bots can be helpful and fast but sometimes we need to talk with a person. Proper design combined with resources provided by some service providers such as Facebook Messenger can provide a seamless transition to a live person if the customer asks for it or indicates frustration with the bot.
Chromis is venturing into messaging bots and plans to release a beta product during 2018. Stay tuned for more news.