ChatGPT is OpenAI's conversational AI assistant and the product that first brought large language models to mainstream, everyday use. It supports text, voice, image, and file input, and can browse the web, run code in a sandboxed environment, and generate images through integrated DALL·E access. A key differentiator is its Custom GPTs system, which lets users and businesses build specialized versions of the assistant configured with their own instructions, knowledge files, and connected tools, then share or sell them through OpenAI's GPT Store. ChatGPT is available as a free tier with usage limits and paid Plus, Team, and Enterprise tiers that unlock higher limits, priority access to newer models, and admin controls.