An API, or Application Programming Interface, is a set of rules and protocols that allows different software applications to communicate with each other. It defines the methods and data formats that applications can use to request and exchange information, without needing to know how the other system is implemented internally.

An API developer is a specialized back-end programmer who designs, builds, tests, and documents APIs. They are responsible for creating the server-side logic that responds to API requests, interacting with the database, and ensuring the API is secure, fast, and reliable.

REST is an architectural style that uses standard HTTP methods (like GET, POST, PUT, DELETE) and a set of predefined endpoints to access resources. It's the most common type of API. GraphQL is a query language for APIs that allows the client to request exactly the data it needs in a single request, which can be more efficient than REST in some cases.

API security is critical because APIs are often the main gateway to a company's sensitive data and business logic. A single vulnerability in an API could allow an attacker to steal customer data, disrupt services, or take over accounts, leading to massive financial and reputational damage.

An API Gateway is a management tool that sits in front of one or more APIs and acts as a single entry point for all clients. It can handle tasks like authentication, rate limiting, caching, and logging, which simplifies the individual APIs and provides a centralized point of control and security.