Infravision is an intrinsic property possessed by dwarves, elves, gnomes, and orcs. Many monsters also have this ability. Humans do not have infravision, to make use of it, they must polymorph into some other creature that has infravision.
Infravision allows a character to detect warm-blooded or fire/ice-based monsters in their line of sight, even if that location is dark. Such monsters are known as infravisible.
Infravision does not detect invisible creatures.