What is GitHub Actions and how does it work?

GitHub Event (push, PR, schedule)
        ↓
Workflow file (.github/workflows/ci.yml) is triggered
        ↓
Runner (Ubuntu/Windows/macOS VM) is provisioned
        ↓
Jobs run (each job gets a fresh runner)
        ↓
Steps execute inside each job (shell commands or Actions)
        ↓
Results reported back to GitHub (pass/fail, logs, artifacts)

Repository
└── .github/
    └── workflows/
        ├── ci.yml        ← runs tests on every push
        ├── deploy.yml    ← deploys on merge to main
        └── nightly.yml   ← runs regression at 2 AM

# .github/workflows/hello.yml
name: Hello World

on:
  push:
    branches: [main]

jobs:
  say-hello:
    runs-on: ubuntu-latest

    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Say hello
        run: echo "Hello from GitHub Actions!"

      - name: Show runner info
        run: |
          echo "OS: ${{ runner.os }}"
          echo "Branch: ${{ github.ref }}"
          echo "Commit: ${{ github.sha }}"