A standard doorway for software to share data and actions.
An API is a drive-thru window for software. You order fries. You do not climb into the kitchen.
AI uses APIs to reach models and work tools. Developers use them to plug those powers into apps.
LLM
An API lets LLM power enter real apps as a service.
Inference
Inference requests often enter a model service through an API.
Function-calling
Function-call uses a set format to trigger the right API.
MCP
MCP can bundle many APIs into tools a model can use.