APIs power modern software. They connect apps, move data, and run agentic AI workflows. But every API is also a door into your systems. Attackers know this. They target APIs more than any other layer today. This guide breaks down API security from the ground up. You will learn what it means, why it matters, and how to protect your APIs against real-world threats. We cover risks, the OWASP API Security Top 10, best practices, tools, and use cases. Let’s get started.