This is taken from the Pottermore website, so look no further if you wish not to know…
The Hufflepuff common room is entered from the same corridor as the Hogwarts kitchens. Proceeding past the large still life that forms the entrance to the latter, a pile of large barrels is to be found stacked in a shadowy stone recess on the right-hand side of the corridor. The barrel two from the bottom, middle of the second row, will open if tapped in the rhythm of ‘Helga Hufflepuff’.* As a security device to repel non-Hufflepuffs, tapping on the wrong barrel, or tapping the incorrect number of times, results in one of the other lids bursting off and drenching the interloper in vinegar.
A sloping, earthy passage inside the barrel travels upwards a little way until a cosy, round, low-ceilinged room is revealed, reminiscent of a badger’s set. The room is decorated in the cheerful, bee-like colours of yellow and black, emphasised by the use of highly polished, honey-coloured wood for the tables and the round doors which lead to the boys’ and girls’ dormitories (furnished with comfortable wooden bedsteads, all covered in patchwork quilts).
A colourful profusion of plants and flowers seem to relish the atmosphere of the Hufflepuff common room: various cactii stand on wooden circular shelves (curved to fit the walls), many of them waving and dancing at passers-by, while copper-bottomed plant holders dangling amid the ceiling cause tendrils of ferns and ivies to brush your hair as you pass under them.
A portrait over the wooden mantelpiece (carved all over with decorative dancing badgers) shows Helga Hufflepuff, one of the four founders of Hogwarts School, toasting her students with a tiny, two-handled golden cup. Small, round windows just level with the ground at the foot of the castle show a pleasant view of rippling grass and dandelions, and, occasionally, passing feet. These low windows notwithstanding, the room feels perennially sunny.
* The complexity or otherwise of the entrance to the common rooms might be said to give a very rough idea of the intellectual reputation of each house: Hufflepuff has an unchanging portal and requires rhythmic tapping; Slytherin and Gryffindor have doorways that challenge the would-be entrant about equally, the former having an almost imperceptible hidden entrance and a varying password, the latter having a capricious guardian and frequently changing passwords. In keeping with its reputation as the house of the most agile minds at Hogwarts, the door to the Ravenclaw common room presents a fresh intellectual or philosophical challenge every time a person knocks on it.
Nevertheless, it ought not to be concluded from the above that Hufflepuffs are dimwits or duffers, though they have been cruelly caricatured that way on occasion. Several outstanding brains have emerged from Hufflepuff house over the centuries; these fine minds simply happened to be allied to outstanding qualities of patience, a strong work ethic and constancy, all traditional hallmarks of Hufflepuff House.