Gaffer and Key Grip are two different departments. It is like comparing the Marketing Department and the HR Department to make it easier for you to understand. They both work for the same thing. They each have a different task. 

You can use Duct Tape for anything you need. The only problem is the stickyness. When you remove it, there is a lot of glue residue on whatever you applied it on. When you are working with expensive gear, you don't want that. So you use Gaffer Tape. It does the same thing as the Duct Tape except it leaves less glue residue on the equipment.

When you work with a small team to make a short film, like you and your friends, you don't have a Key Grip and you might not know what it is. When you work with a big team, you have so many new roles that you might have no idea about. Key Grip is one of them and it is also very important. He is the boss of the grip department and he helps with the gear. I mean they literally move the tripods, dolly...