Sight is a sense that most animals use in the struggle to survive. Whether through predation, procreation, or movement, sight is usually the primary tool that animals rely on. The visual spectrum depends on standard light to work, but infrared sight uses heat as the primary source of vision. Some animals can use the infrared spectrum to "see."
Snakes such as boa constrictors and pit vipers use infrared to see their prey. In any kind of lighting or weather condition, these snakes can see the body heat coming off their victims through the infrared spectrum, which allows them to strike with deadly accuracy. Some snakes, such as boas, can actually see the world in two ways: through normal and infrared vision.
Some species of fish, such as piranha and goldfish, can see in infrared light. This is useful to them in murky waters, where the shorter rays of the color spectrum can be broken and diffused, whereas the longer infrared rays can go further in the water. Many of these fish also see the world in color.
The bloodsucking mosquito uses infrared to hone in on the blood rich parts of its prey, allowing it bullseye skill to suck the blood and get out before being noticed. Mosquitoes primarily use sight and smell until they get within 10 feet of their prey, at which point heat vision takes over.