House-Elves are a fictional sapient species seen in the Harry Potter series.
House-Elves are relatives of Redcaps and Erklings, as well as, more distantly, Goblins. Unlike them, however, House-Elves were magically bound into slavery by British wizards (although this is apparently not the case in Peru), some of whom took the opportunity to horribly abuse their House-Elves, forcing them to punish themselves physically over all sorts of 'offences' such as speaking out of turn.
This has gone on for so long that most House-Elves accept this as a status quo and would never dream of freedom, some being even repulsed by the thought. At any rate, House-Elves are freed from their bondage if their master gives them clothes (though whether or not they do so willingly doesn't matter); because of this, House-Elves do not normally wear conventional 'clothes', covering themselves with various items like pillowcases.
House-Elves possess a type of magic different from that of regular wizards, such as an ability to teleport which standard anti-teleportation spells do not cover.