What are the twelve constellations?
What do you mean "the" 12? There are not 12 constellations, there are a total of 88 constellations. Perhaps you are confusing with the signs of the zodiac.
Astronomically, there are thirteen, arguably fourteen zodiacal constellations with Ophiuchus joining the classical twelve and Cetus's constellation boundary's corner touching the ecliptic. Astrologically, there are only twelve zodiac signs. Signs and constellations are not the same thing, but they are named after one another.
There are 88 "officially recognized" constellations, according to the International Astronomical Union, the IAU. Everybody will have their own favorites, of course, but in my opinion the top two "most commonly known" constellations include the Big Dipper and Orion the Hunter. Beyond that, there are twelve "signs of the Zodiac", constellations that lie along the ecliptic.