Fine-tuning is a process of teaching a large language model (LLM) to behave in a certain way, typically through supervised fine-tuning, where examples of desired responses are provided. It's similar to training a new employee, with the LLM starting with broad understanding and being trained on specific scenarios to handle common inputs. Fine-tuned models excel at learning desired behavior, developing expertise in a subject, consistency, speed, and cost, but struggle with handling out-of-domain inputs and deep reasoning ability. They're particularly useful for tasks where a model needs to be highly specialized and efficient, such as chatbots, data analysts, and summarizers, offering significant cost savings compared to using a general-purpose LLM like GPT-4. However, they may not be suitable for high-volume or rapidly changing use cases, and their effectiveness depends on the quality of the training data.