Mount Everest is the highest summit on planet Earth, when measuring from sea level. The Everest is located within the Himalaya mountain range, on the border of China and Nepal. The summit measures 8848 meters from sea level.
It is known since the mid 1800's, that Everest is the highest peak of the planet, the first measurement happened in 1856, claiming a height of 8840 meters, which equals roughly 29000 feet.
Though the Everest is generally accepted that the "highest" mountain, it is better to know that there can be different approaches to the question, which gives different answers to the question of which is the highest summit on Earth. Measuring from the ocean bottom Mauna Kea (in Hawaii) is higher than Everest with a total elevation of 10 200 meters (~33000 feet). Another summit that can be considered as "highest" is Mount Chimborazo in Ecuador, which is the one whose summit is farthest away from the Earth's center.