GPT (Generative Pre-trained Transformer) models are large language models developed by OpenAI that use transformer architecture to generate human-like text, analyze images, process audio, and perform complex reasoning tasks. From GPT-4's multimodal capabilities to GPT-5's enhanced reasoning and extended context windows, these models power applications ranging from chatbots to code generation and document analysis. Understanding their architecture, tokenization mechanisms, and API parameters is essential for leveraging their full potential, as choices around temperature, context windows, and model selection directly impact output quality, cost, and latency in production systems.