A limited series is a comic with a fixed amount of issues.
In 1979, Dave Sim was hospitalized, due to overdose of grass and LSD. During this time, he came up with the idea for Cerebus the Aardvark and to limit it to 300 issues. Cerebus was the very first limited series (although somehow Dave failed to realize the latter).
DC Comic, which was suffering from the damage of the DC Implosion, decided to experiment with this style of story telling, which proved successful. Later, Marvel Comics and Dark Horse Comics adopted the style of story telling.
Although 4 to 6 issues is the norm, the number of issues can vary. Gen13 ran for 4 issues, while Squadron Supreme and Watchmen had 12 issues. Usually, an issue of a limited series will say on the cover what the limit of issues is.