API (Application Programming Interface)

APIs are the building blocks of modern software development. They enable applications to leverage functionality from other services without understanding their internal workings.

Types of APIs: - REST APIs: Stateless, HTTP-based, most common for web services - GraphQL: Query language allowing clients to request specific data - SOAP: XML-based protocol, common in enterprise systems - WebSocket: Real-time, bidirectional communication - gRPC: High-performance, binary protocol for microservices

Key API Concepts: - Endpoints: URLs where API requests are sent - Authentication: API keys, OAuth, JWT tokens - Rate Limiting: Restrictions on request frequency - Versioning: Managing changes without breaking clients

API-First Development: Modern companies build APIs as products. Stripe, Twilio, and Plaid became billion-dollar companies by providing excellent APIs that developers love to use.

For Founders: APIs can be your product (Stripe), distribution channel (Shopify), or integration strategy. Understanding APIs is essential for technical and non-technical founders alike.