ChatGPT is a language model developed by OpenAI, and it is based on the GPT (Generative Pre-trained Transformer) architecture. Specifically, it is built on the GPT-3.5 architecture. GPT-3 stands for "Generative Pre-trained Transformer 3," and it is the third iteration of the GPT series. The model is trained using a vast amount of diverse data from the internet, allowing it to generate human-like text based on the input it receives. It has 175 billion parameters.