Coin

As the name implies, these are coins made out of gold. They are found all over the Mushroom Kingdom.

In the video game Mario and Luigi: Superstar Saga, in the BeanBean Kingdom, they are known as Mushroom Coins and are worth far less than BeanBean Coins.

Also, in most of the Super Mario World games, 100 Gold Coins can be traded in for an extra life.

In Super Mario 64, 100 of these coins will grant Mario a Power Star.

In Super Mario Sunshine, 100 of these coins will grant Mario a Shine Sprite.

Gold coins are used as currency to buy items (including a Star) in the Mario Party series. You collect these coins by landing on coin squares, winning them after mini-games, winning the lottery, etc. At the end of the Mario Party tournament there is a chance you will recieve a bonus Star if you have the most coins out of all the contestants.