GitHub Actions is a popular CI/CD platform used in modern software development. It allows users to run pipelines to build, test, lint, and deploy their code. Runners are the apps that execute these pipelines. Self-hosted runners provide more customization and control for users but require manual work and self-management, which can result in higher costs. GitHub-hosted runners simplify the process by handling most of the configuration and maintenance work but have limitations in terms of OS and hardware selection. The choice between self-hosting or using GitHub's hosting service depends on factors such as team size, specific use cases, and security requirements.