What is the difference between parallel jobs and sequential jobs in a pipeline?

In a CI/CD pipeline, jobs can be structured to run sequentially (one after another, where each job depends on the previous succeeding) or in parallel (simultaneously on separate runners). Sequential jobs are appropriate when there is a logical dependency — you cannot deploy before the build succeeds, and you cannot run integration tests before the unit tests pass. Parallel jobs dramatically reduce total pipeline time by running independent tasks simultaneously — for example, running unit tests, linting, and security scanning at the same time rather than waiting for each to finish. In GitHub Actions, jobs run in parallel by default; use needs: to express dependencies. A well-designed pipeline parallelizes as much as possible while maintaining the correct logical ordering, minimizing the time developers wait for feedback.