Homenum revelio is a spell used to detect human presence. When the spell finds someone, the person whose presence has been revealed feels as if "something [is swooping] low over him [or her], immersing his [or her] body in shadow" (DH9, DH21).
Albus Dumbledore sometimes uses Homenum revelio non-verbally on occasions when Harry Potter is concealed by his Invisibility Cloak, giving the Harry the impression that the Headmaster can see him through the Cloak (BLC, CS14, PS12).
Hermione Granger uses the spell to confirm that number twelve, Grimmauld Place is deserted when the Trio arrives at the house after Bill and Fleur Weasley's wedding and decides to use the old house as a hideout (DH9).
Travers casts Homenum revelio while in Xenophilius Lovegood's house to find out whether Mr. Lovegood's is telling the truth about Harry Potter's presence in his house. Due to the Death Eater's affirmation, after using the spell, that there is "someone up there", when there were in fact three people, it is possible to assume that Homenum revelio does not reveal specifics about the people whose presence the spell detects (DH21).