A chatbot is a program that interacts with users through chat interfaces like messaging apps. Chatbots are simple and inexpensive to develop and deploy, and allow asynchronous notifications and integration with teams. To build a chatbot, you define a grammar for commands, program actions for the bot to take, and define how it will return results to the user. Advanced features include integrating webhooks and slash commands for more capabilities. Security measures like restricting commands and using tokens are also important to implement.