Open source AI models — such as Meta’s Llama, Mistral, and Falcon — release their model weights and architecture publicly, allowing developers to run, fine-tune, and deploy them without API costs or usage restrictions. This contrasts with proprietary models (GPT-4, Claude, Gemini) that are accessed via paid APIs. Open source models enable organizations to self-host AI for privacy-sensitive applications, reduce costs at scale, and customize model behavior through fine-tuning on proprietary data.