How to adjust the water level in a toilet tank

Questions covered in this video:

1. How do I adjust the water level in my toilet tank?
2. What is the fill valve and where is it located in the toilet tank?
3. What is the purpose of the float in the toilet tank?
4. How do I lower the water level in the toilet tank?
5. How do I raise the water level in the toilet tank?
6. What tool do I need to adjust the fill valve?
7. How do I know if the water level is correctly adjusted?
8. What is the significance of the line marking in the toilet tank?