The Ticket API is the one most people reach for first: create a ticket, update its status and fields, add a comment, attach a requester. It's how a web form, an order system, or a monitoring tool turns an event into a ticket without a human typing it in.
The Search API lets you query tickets, users, and organizations with filters, useful for pulling a specific slice of data rather than everything. For bulk exports, the Incremental Export endpoints give you a cursor-based way to pull large volumes of records (tickets, users, organizations) without re-downloading what you already have.
Beyond tickets, the API covers users and organizations (create, update, merge, tag), macros and triggers (read and sometimes manage them programmatically), and the Help Center API for articles. If you want to build something that lives inside the agent's screen itself, like a custom sidebar panel, that's the Zendesk Apps Framework (ZAF), a separate toolkit for building apps that run in the agent workspace, whether private to your account or published to the marketplace.
Authentication is usually an API token generated in Admin Center (under Apps and integrations), though OAuth is the better fit if you're building something end users will authorize themselves. Every account has rate limits, and they scale with your plan, so a bulk sync that works fine at 50 requests a minute can start throttling at higher volume if you're not pacing it.