The Nimbus 2000 is a high-end broomstick produced by the Nimbus Racing Broom Company.
Harry's first broom was a Nimbus 2000. It was given to him by Professor McGonagall in his first year (PS10). At the time he received it, it was the best broom available; he used it until it blew into the Whomping Willow during his third year and was destroyed (PA9).
The following model, the Nimbus 2001 was released just before the start of Harry's second year in 1992. Lucius Malfoy bought seven of these to outfit the Slytherin Quidditch Team; there's some evidence that this was why Draco was made the team's Seeker.
After losing his Nimbus 2000, Harry received a Firebolt for Christmas from his godfather, Sirius Black (PA11).
The word "nimbus" most commonly refers to clouds that precede rain (source: https://en.wikipedia.org/wiki/Nimbus). -BB
Nimbus racing brooms may also take their name from two racehorses, one of which won the Epsom Derby in 1949 (source: https://en.wikipedia.org/wiki/Nimbus). -BB