An atmosphere is a body of gas which can exist around a celestial body, such as a planet or moon. For example, the atmosphere of the Earth is composed of 78% nitrogen, 20% oxygen, 1% argon and trace amounts of other elements. On the other hand, the atmosphere of Jupiter is primarily composed of hydrogen. Atmospheres are held in place by the planet's or moon's gravitational force, and the constituents of the atmosphere often depend on the strength of gravity in question. For example, when the Earth first formed its atmosphere mostly contained hydrogen and helium, but the gravity was not strong enough to hold onto these light gases and they were lost into space. The present atmosphere is composed of the heavier elements listed above. Jupiter on the other hand has a very powerful gravity, due to its large mass, and is capable of holding onto hydrogen.
The atmosphere of Earth is subdivided into multiple layers, the lowest of which is the troposphere, in which humans live. Most commercial aircrafts fly within the troposphere. Next up is the stratosphere. Concorde flew in this layer, as do weather balloons. Above that is the mesosphere. The troposphere, stratosphere, and mesosphere together make up the homosphere, in which the components of the atmosphere are well mixed. Above the homosphere is the heterosphere, where the molecular mean free path is large enough that the relative molecular weights of atmospheric gases affects their distribution.
The official boundary to space is encountered at 62 miles (100 km) of altitude, although the United States of America sometimes defines the edge of space to be at 50 miles (roughly 80 km). In actual fact the atmosphere extends considerably beyond both of these altitudes. The International Space Station is affected by atmospheric resistance as it orbits the Earth.
Other atmospheres in the Solar System
Mercury has very little atmosphere, constituting little more than trace amounts of gases. Venus, on the other hand, has an intense Sulfur Dioxide atmosphere which is believed to have caused a runaway greenhouse effect, leading to the lethal pressures and temperatures seen on that planet today. Mars' atmosphere is far thinner than Earth's, it has little in the way of clouds and most martian weather constitutes the dust storms which sometimes encircle the entire planet. The gas giant planets, Jupiter, Saturn, Uranus and Neptune have more hydrogen and helium in their atmosphere. Being further out from the sun they received large amounts of hydrogen when the sun went nuclear 5 billion years ago and blew much of the hydrogen out of the inner solar system. Saturn's moon Titan also has an atmosphere, which contains large volumes of clouds, making the world extremely difficult to study without landing there. For that reason the Cassini mission included a lander which landed on Titan, sending back images and data from the surface.