The Inferi are animated corpses who do the bidding of the Dark wizard who created them, like gruesome puppets (HBP4, HBP21). The singular form of the word is Inferius (HBP3, HBP9, HBP21).
Harry encountered a large number of Inferi floating just below the surface of the underground lake surrounding the cursed basin where the locket Horcrux had been hidden. The Inferi relentlessly attacked Harry when he touched the water but Dumbledore surrounded them with a ring of fire which drove the Inferi away, back into the water (HBP26).
Regulus Black and Kreacher also encountered the Inferi when they retrieved the locket (prior to Harry and Dumbledore's visit).
In Defense Against the Dark Arts class, Seamus asks Professor Snape the difference between an Inferius and a ghost. Snape prompts Harry, who has been talking, to answer the question. Struggling, Harry gives the iconic practical-yet-obvious answer that ghosts are transparent.
The Haitian Quidditch team use Inferi as their mascots (DP).
Commentary
Etymology
'inferi,' Latin 'the dead'
This is also similar to the Latin word 'inferus,' which means 'underneath' or 'below'