NASA is an agency of the United States of America. That means it is part of the administration of this country. NASA explores space and wants to learn something for life on Earth. NASA is known, for example, for putting people on the moon.
The abbreviation NASA stands for "National Aeronautics and Space Administration". Translated, it means something like "National Aeronautics and Space Administration". In the USA, the Air Force used to take care of space issues. Since 1958, NASA has been responsible for this, i.e. an agency that no longer belongs to the military.
At that time, the USA started sending people into space. The Soviet Union was always a few steps ahead of the USA: Yuri Gagarin was the first man in space in 1961, barely a month before the first American, Alan Shepard. In the same year, American President John F. Kennedy proclaimed the goal: The US should put a man on the moon and bring him back. The Apollo programme actually achieved this in 1969.
Other NASA projects were not as successful. One example is the Space Shuttle, which was used from 1981 onwards. It was supposed to carry satellites and people into space more cheaply than rockets. In fact, the Space Shuttle flights became much more expensive than planned. In 2011, therefore, the last Space Shuttle flew to the International Space Station.
But NASA has also launched many unmanned space probes into space that have explored the solar system and its planets. The Voyager probes have even left our solar system.
Human spaceflights are particularly expensive. Even in the time of the moon landing, many Americans complained that all that money would be better spent on other things. Therefore, NASA has to find good reasons for its work. For example, it studies the earth's climate with its satellites.