CircleCI is a container-native CI platform. Pipelines are defined in .circleci/config.yml using a YAML model that distinguishes jobs (units of work), steps (commands within a job), and executors (compute environment: Docker, machine, macOS, Windows, GPU). Jobs within a workflow run in parallel or in dependency order.
Orbs are reusable, versioned packages of CircleCI configuration that encapsulate jobs, commands, and executors, distributed via the Orb Registry. Docker Layer Caching stores intermediate image layers between runs. Test splitting distributes test files across parallel containers based on historical run-time data so all containers complete at approximately the same time.
Resource classes configure compute per job (CPU, memory, OS), enabling jobs with different compute requirements to run on appropriately sized machines within the same workflow. A self-hosted runner option and an on-premises server product cover isolated environments. Pricing is credit-based; credits are consumed at different rates per resource class. Free tier: 6,000 credits/month on Linux.