What is an inferi?

Inferi are dead bodies that have been reanimated by a Dark wizard. Inferi are described by Professor Albus Dumbledore in Chapter Four as "[d]ead bodies that have been bewitched to do a Dark wizard's bidding." Professor Dumbledore also informs Harry that the Inferi have not been seen since Voldemort's original rise to power several years prior, when Voldemort created many Inferius.
Further information about the Inferi is revealed by Professor Severus Snape in Chapter 21, when he explains to the class that an Inferius is not alive, but merely reanimated to act as "a puppet to do the wizard's bidding."
In Chapter 26, as they attempt to retrieve the supposed Horcrux from inside the seaside cave, Harry and Professor Dumbledore encounter Inferi. An Inferius jumps from the water when Harry tries to use the "Accio" spell to find the Horcrux. As they approach the island in a boat, Harry sees "a dead man lying faceup inches beneath the surface, his open eyes misted as though with cobwebs, his hair and his robes swirling around him like smoke." Professor Dumbledore explains to Harry that the Inferi were likely placed there by Voldemort to defend the Horcrux. It is in this chapter that readers also learn that Inferi fear light and warmth.

Inferi is the plural form of an inferius, a term introduced in Harry Potter and the Half-Blood Prince.
In Chapter 4, Dumbledore answers Harry's question regarding inferi：
“They are corpses,” said Dumbledore calmly. “Dead bodies that have been bewitched to do a Dark wizard’s bidding. Inferi have not been seen for a long time, however, not since Voldemort was last powerful. . . . "
In Chapter 21, Snape explains to the students the difference between an inferius and a ghost:
“The Inferius is a corpse that has been reanimated by a Dark wizard’s spells. It is not alive, it is merely used like a puppet to do the wizard’s bidding. A ghost, as I trust that you are all aware by now, is the imprint of a departed soul left upon the earth . . . and of course, as Potter so wisely tells us, transparent.”
In Chapter 26, Harry Potter and Albus Dumbledore encounter inferi as they attempt to destroy Voldemort's locket horcrux, which is believed to be on an island. Inferi show up from below the water, dragging Harry down in order to drown him. Harry tried several spells, but they don’t work that well on inferi. Dumbledore conjures fire to get them out of inferi. Fire is the most effective way to defend yourself against inferi.