"You will have classes with the rest of your house, sleep in your house dormitory, and spend free time in your house common room."
-- Professor McGonagall to the First Years before Sorting (PS7)
The common room in a school is a room to which all members in a certain category have common access for social or business purposes. At Hogwarts, each of the Houses has a common room where the students of that House congregate for meetings, to study, and to socialize. A notice board in each common room displays not only announcements of upcoming school events but also personal notes about lost books or chocolate frog cards to trade.
Commentary
Notes
Rowling discussed the symmetry of the common rooms in an interview:
It is the tradition to have four houses, but in this case, I wanted them to correspond roughly to the four elements. So Gryffindor is fire, Ravenclaw is air, Hufflepuff is earth, and Slytherin is water, hence the fact that their common room is under the lake. So again, it was this idea of harmony and balance, that you had four necessary components and by integrating them you would make a very strong place. But they remain fragmented, as we know.
Readers of Dorothy L. Sayers' novel 
Gaudy Night which is set at Oxford University, will have encountered the term there. In the Muggle world, the word can also be used to refer to the people who use the room as well as the room itself, but the Harry Potter books have not used it like that.
From the Web
Listen to fan-created ambient sounds of each of the Hogwarts Common Rooms on Ambient-Mixer.com
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