API (application programming interface)

A client sends a request to an endpoint, a URL such as /v1/contacts, using a method defined by the HTTP standard: GET to read, POST to create, PUT to replace and DELETE to remove. It includes credentials, such as an API key or OAuth token, and usually a JSON body. The server answers with a status code and data.

REST is the most common style for these APIs: stateless requests on resources that each have their own URL. Other styles include SOAP, GraphQL, gRPC and WebSockets, and webhooks invert the direction by having the server push events to you. APIs are also grouped by access: public, private and partner.

For example, a RevOps engineer writes a nightly script that asks the outreach tool's API for every reply marked interested in the last 24 hours, about 35 on an average day. It then creates or updates each contact and a deal through the CRM's API, about 120 requests a night, well under both tools' rate limits.

APIs are how a sales stack fits together. Native integrations between a CRM and an outreach tool are API calls underneath, and when no integration exists, an API lets RevOps or a GTM engineer build one. Watch the limits in practice: rate limits cap requests per minute, and pagination splits large results into pages. AI agents use APIs too, often through MCP servers that wrap an API in tools a model can call.