Glob finds files by name pattern. src/**/*.ts returns every TypeScript file under src/. **/*.test.js finds all test files regardless of depth. Use Glob when you know what the file is called but not where it lives.
Grep searches file contents with regex. console\.error across src/ finds every error log. TODO|FIXME shows all flagged comments in the codebase. Grep returns matching lines with file paths and line numbers. I'll often use Glob first to narrow the file set, then Grep within those files to find the exact line needed.