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Understanding the `if __name__ == '__main__':` idiom

I've seen many examples of Python scripts that include a line that says:

```python
if __name__ == '__main__':
```

Sometimes the following block contains a bunch of code, but other times it just makes a single function call (like `main()`), or some variant along the lines of

```python
import sys

# other stuff here...

def main(args):
    ...

if __name__ == '__main__':
    sys.exit(main(sys.argv[1:]))
```

I can see that this must be checking the value of some `__name__` global variable - but that isn't defined anywhere in the code.

Why would there be such a variable, and why might it be equal to `__main__`? What is the purpose of doing this check, and why might I include it in my own code?