ChatGPT uses a model that was trained on 570GB of data (300 billion words), mostly on data crawled from the internet, Wikipedia, and books.
GPT-3.5 has 175 billion parameters, which pales in comparison to the 1.7 trillion parameters GPT-4 has.
Parameters are settings that give fine-grained control over the content in AI responses, including the tone, style, and creativity.
Source: Open AI Blog