A more elaborate explanation of this subject can be found here, on Harry Potter Wiki.

A house-elf, sometimes referred to as elf though not to be mistaken with elves, is a small creature with magical powers. House-elves are known to be very devoted to their masters, unless they are freed, which can be done through giving them a piece of clothing. They are between two and three feet tall, with spindle arms and legs and a big head and eyes.

Notable house-elves

  • Dobby
  • Kreacher