ChatGPT is a free interface launched by OpenAI in late 2022 that lets you chat with an LLM. You type a question, it answers. It doesn't think, it predicts text that seems right, based on what it learned during training. It's powerful for writing, summarizing, translating, coding. But it can be wrong with confidence (the famous "hallucinations") and doesn't know what happened after its training cutoff.
Key takeaway: ChatGPT is not a search engine. For recent or critical facts, always verify.