What is an API and do I need one?

An API (Application Programming Interface) allows different software to communicate. You interact with APIs constantly—when an app shows weather data, it's calling a weather API. You need to build an API if you're creating a service others will integrate with, or if you're separating your frontend from backend. For simple websites, you probably don't need to build one, but you'll likely use existing APIs (payment processors, email services, etc.).