API (Application Programming Interface)
An API (Application Programming Interface) is a set of rules and protocols that allows different software applications to communicate with each other — like a contract defining how one piece of software can request data or actions from another.
In depth
APIs are the backbone of modern software. When your app shows a Google Map, processes a Stripe payment, or sends a Twilio SMS, it's using an API.
Types of APIs: - REST API: the most common — uses HTTP methods (GET, POST, PUT, DELETE) and returns JSON - GraphQL API: client specifies exactly what data it needs — more flexible than REST - Webhooks: event-driven — a server sends data to your app when something happens - WebSocket: persistent connection for real-time data (chat, live feeds) - SDK: a library that wraps an API for easier integration (Stripe SDK, Supabase client)
For founders, APIs matter because: - You can build faster by using existing APIs (payments via Stripe, auth via Clerk) - Your own API lets mobile apps and third parties integrate with your product - API access is often a high-value feature for B2B SaaS customers
Real example
When you book a flight on Expedia, Expedia's backend calls the airline's API to check availability, the Stripe API to process your payment, the Twilio API to send an SMS confirmation, and Sendgrid's API to send a confirmation email — all in under 3 seconds.
Tools & calculators
Related terms
PRD (Product Requirements Document)
A Product Requirements Document (PRD) is a document that describes what a product or feature should do — the problem it solves, who it's for, and what functionality it needs — before development begins.
SaaS (Software as a Service)
Software as a Service (SaaS) is a software distribution model where applications are hosted in the cloud and provided to customers over the internet on a subscription basis, rather than installed locally.
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