So any piece of software that does an automated task is called a bot. Bots which interact in the form of a conversational interface are called chat bots. They might live outside chat platforms or inside some chat platforms.
In common cases, a chatbot usually 'talks' to a human. And based on what the user says, chatbots performes some action or replies accordingly.
A chatbot might talk in vairety of ways including text or voice. And it is not required that how chat bot talks is indistibugishable from how a human talks. They don't need to pass 'Turing's test'.
Chatbots follow a conversational interaction design pattern which focusses on natural seeming input syntax and has very minimal clutter of UI elements. The flow of fucntion desiged for conversational paradigm abstracts away uncessary details and breaks things down as much as possible for the end user as compared to conventional GUI.
Chat bots care great for increasing sales , especially in B2B industry where the products/services are complex and need customization.
Chatbots can be used for :
-Capturing New Leads
-Conducting A Survey
-Providing Customer Support
-Selling New Products
Chatbots are actually mini apps which can serve variety of use cases.