It's a common misconception, but the Big Dipper is actually an asterism. Asterisms are defined as group of stars that are easily recognizeable from Earth. Asterisms may be a subgroup of stars from a bigger constellation, or include stars from multiple constellations.
In the case of the Big Dipper, the 7 star asterism is only part of the Ursa Major constellation. The reason it's so easily recognizeable is because the Big Dipper's stars are the brightest among all the stars that make up the Ursa Major. Because they are very easy to spot, they have aided in navigation throughout the year.
If you're curious, the International Astronomical Union has officially recognized 88 constellations since 1922. 48 of them were first recognized by Ptolemy in the 2nd century AD.