"These belong in Hufflepuff,
Where they are just and loyal.
Those patient Hufflepuffs are true
And unafraid of toil."
-The Sorting Hat, singing it's start-of-term song for the 1993-94 school year (PS7)
This is starting to feel like the dawn of the Age of Hufflepuff.
-- JKR on Twitter
Hufflepuff House is the Hogwarts house of founder Helga Hufflepuff. Hufflepuff House values character traits such as dedication, fair play, and hard work. Hufflepuffs are often unjustly characterized as unintelligent or as losers not good enough for any of the other houses.
Head: Professor Pomona Sprout (1990s)
Founder: Helga Hufflepuff
Blazon: or, a badger sable (that is, a black badger on a golden field, hence the Hufflepuff colours of yellow and black) (GF15)
Ghost: The Fat Friar
Location: Accessed through a pile of barrels that is in a recess a little past the still life painting that is the entrance to the kitchens (Pm); to reach it, you enter a door to the right of the main staircase in the Entrance Hall and down a staircase there (GF17). Although it is therefore at dungeon level, it is "as dissimilar as possible" from the Potions classroom. The common room is decorated in House colours, which in this case means that there are lots of yellow hangings; like Gryffindor Tower, it has a lot of comfortable armchairs. There are "little underground tunnels leading to the dormitories, all of which have perfectly round doors, like barrel tops" (BLC, JKR).
The stones in Hufflepuff's house points hourglass are diamonds (JKR:Tw).
Although ultimately sorted into Gryffindor, Neville Longbottom asked the Sorting Hat to place him in Hufflepuff, feeling that that house's reputation as a "load o' duffers" better suited his character.
One gains entry to the Hufflepuff common room by tapping out the rhythm to the name "Helga Hufflepuff" on a stack of barrels. Tapping incorrectly or on the wrong barrel will cause one of the barrels to spill vinegar all over the person trying to enter (Pm).
"Huffing and puffing" suggests hard work. "Puff" also makes sense in regards to the stereotype of Hufflepuffs being soft.
There are "little underground tunnels leading to the [Hufflepuff] dormitories, all of which have perfectly round doors, like barrel tops" (BLC, JKR). This last detail is reminiscent of hobbit architecture as described in J.R.R. Tolkien's book The Hobbit. - MLW
The screen capture from HPM lists Corner and Goldstein as Hufflepuffs, but they appear in OP as Ravenclaws; OP is the definitive source, so they do not appear on this page.
Rowling has stated that the four Hogwarts houses "correspond roughly to the four elements" and that Hufflepuff is aligned with earth (TLC). This makes sense as the element of earth is associated with characteristics such as work, substantiveness, and dedication. -BB
It is a reflection of Hufflepuff House's nature that its entrance never changes and requires one to tap a rhythm. -BB
Rowling planned on having Harry visit each house's common room, but she never found an opportunity for him to go to the Hufflepuff common room.
In her annotated copy of PS, Rowling writes that she had originally considered choosing the bear to represent Hufflepuff House, and she now wonders whether the house would get more respect if she had stuck with the bear (source: The Guardian).
Rowling has tweeted that her agent, Neil Blair, is a Hufflepuff (JKR:Tw).
Rowling told Snitchseeker that Hufflepuff was her favorite house in many regards, and that, when given the option to fight in the Battle of Hogwarts or leave, virtually every of-age Hufflepuff stayed to fight (while the Slytherins left and the Ravenclaws were split about 50/50).