In early November 2023, OpenAI announced the release of GPTs and Assistants, which are two products that allow developers to extend and build upon the GPT-4 LLM model. Put simply, they allow you to do all sorts of cool stuff not previously possible.
GPTs are availble only through ChatGPT app and website, and only for subscribers, which means they are best for internal or personal use.
Assistants can be intergated into your own website or app, so they are better at the moment for customer-facing products. They are also a little more complex to develop, and there is a cost per use.