ChatGPT is a variant of the GPT (Generative Pre-trained Transformer) language model that has been trained specifically for conversational text generation. It is built using the same architecture as the original GPT model, but with a smaller number of parameters and a dataset that is more focused on generating human-like text in response to conversational prompts.

ChatGPT has become popular because it is able to generate human-like text that is coherent and contextually appropriate in a wide range of conversational scenarios. This makes it useful for a variety of applications, including chatbots, virtual assistants, and dialogue systems. ChatGPT is also relatively simple to fine-tune for specific use cases, which has made it easy for developers to create new conversational applications quickly.

Another major reasons of its popularity is the availability of pre-trained models provided by openai on GPT-3 which allowed many user, researcher to try the model on their own data, and having those models fine-tune to their use-cases which was not a case before.

Finally, it is relatively simple to use, as it can be integrated into a variety of different platforms and frameworks with minimal modification, and can be easily fine-tuned using a smaller dataset.

Overall, ChatGPT's ability to generate human-like text, its ease of use and adaptability are the main reasons why it has become so popular among researchers, developers and companies in recent years