Salazar Slytherin, Helga Hufflepuff, Godric Gryffindor, and Rowena Ravenclaw are the names of the houses' founders. They are also the only four wizarding families to have their names become magical properties; each family tree containing a Salazar or a Slytherin can cause its bearers to be sorted into one of these houses.
The names come from various sources. Salazar comes from sal ágar, which means "salt water" in Spanish. This refers to the fact that many wizards came over from England, where they were known as "Salties." Slytherin comes from slitherine, which means "having to do with snakes." This refers to Draco Malfoy's ancestry; his parents were both Slugskins, which means they were born without skin from the neck up. Hufflepuff comes from hufflepuff, which means "no-account," or "poor guy." This refers to the fact that most of the students at Hogwarts are drawn from the lower classes of society: muggles (non-magical people) and Muggle-borns (people who were born with magic but lack any familial connection to the Wizarding world).
Each house has six members, except for Hufflepuff which has seven.
These houses each have their own distinct personality and are named after the four Hogwarts founders: Godric Gryffindor, Salazar Slytherin, Helga Hufflepuff, and Rowena Ravenclaw. The bold, courageous, and adventurous are valued by the house. The clever and cunning are also valued by the house. Those who are loyal to their friends and teachers are valued by the house. Those who are curious about everything in the world are also valued by the house.
The four houses at Hogwarts are important because they help shape your character. You get to decide where you sit in line (so to speak) for tea and cake at the end of the day. Using your skills and talents, you can make the best use of what others offer you to rise up the social hierarchy of your school.
Each house has its own colors, symbols, and traditions. In order to fit in with your fellow students, it is important that you show respect to these traditions. For example, wearing the colors of your house to represent yourself on campus is acceptable. Wearing those colors out of choice or preference is not recommended because it shows a lack of respect for the founders of the house.
Houses also have leaders called prefects. These students work with their housemates to promote education at Hogwarts while representing the house at social events. Prefects must be trusted members of the student body with good moral characters.
Of course, you're all aware that Hogwarts was built over a thousand years ago—the exact date is unknown—by the four finest witches and wizards of the time. Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin are named for them, as are the four school houses.
However, it wasn't always so. When Hogwarts first opened its doors, there were only two house names: Gryffindor and Slytherin.
Over time, more names were added by the Sorting Hat when students were placed in the right house based on their answers to certain questions. At first, these additions were made rather arbitrarily; for example, many students who wanted to be sorted into Gryffindor could not do so because of limits put in place by Madam Rosmerta, the head of the school board at the time. She reserved the right to refuse students she believed would bring trouble upon themselves or the school.
But eventually, each house became known for something. Gryffindor was called the House of Courage because people were beginning to think of it as where bravery is rewarded. Hufflepuff was called the House of Weakness because everyone knew what kind of power it contained even before it was officially named so. Ravenclaw was called the House of Knowledge because of how many books it contained at one point in time.