It's a group of countries whose leaders try to tackle global problems.
They get together to discuss the issues and then plan what action to take. The '8' members are seven of the world's rich nations, and Russia.
The leaders of these countries take it in turns to be president of the G8. They meet face-to-face at an annual summit which is held in the country which has that year's presidency.
Although everyone gets a chance to be president, the USA is always the richest and most powerful member. That means their decisions tend to be the most important.