ChatGPT is a Large Language Model (LLM) built for conversation. It is a successor to InstructGPT. Both these models are built by OpenAI. I won't waste your time with another article about its uses or examples of how it performs.
These models are chatbots (the ability to converse with humans and maintain a dialogue), which are active areas of Natural Language Processing (NLP) research. There have been quite a few notable attempts (Tay from Microsoft, for example) which failed spectacularly after being left to train with open data from Twitter.