To decide is to make up your mind to do (or not do) something. People decide hundreds of things a day, from what to eat to what TV shows to watch.
Deciding involves making decisions — choosing what to do. There are a lot of things in life to decide, such as where to live, what school to go to, where to apply for a job, and what kind of clothes to buy and wear. At any restaurant, there are hundreds of options: you have to decide which is right for you. You can also say that if a touchdown ended a game, it decided the game.