The Slack Web API is the REST surface for interacting with a Slack workspace: posting messages, managing channels, uploading files, querying users. It pairs with the Events API (real-time push) and Socket Mode (a WebSocket transport that avoids public callback URLs) to form the full app-development stack.

Auth is OAuth 2.0 with fine-grained scopes — each operation requires its specific scope (chat:write, channels:read, files:write, etc.), confirmed by an admin at install time. Bot tokens (app acting as itself) and user tokens (app acting on a user's behalf) are deliberately separate.

Block Kit is Slack's message-layout spec — buttons, menus, images, and form fields render consistently in messages. Combined with Modals and the Home Tab, external apps can feel almost native. Free workspaces gate some advanced methods (like long message history), and rate limits split methods into tiers — hot ones like chat.postMessage are tightly throttled, so plan capacity ahead of time.