ChatGPT is an advanced conversational AI developed by OpenAI. It is built upon a Large Language Model (LLM) architecture, specifically the Generative Pre-trained Transformer (GPT) series. LLMs are deep learning models trained on vast amounts of text data, allowing them to understand, generate, and respond to human language with remarkable coherence and context. They function by identifying complex patterns and relationships within language, enabling them to predict the most probable sequence of words to form meaningful sentences or paragraphs. Natural Language Processing (NLP) is the field of AI that enables computers to understand, interpret, and manipulate human language, forming the backbone of LLM capabilities.