Alex Nanson already gave the answer.
I just wanted to emphasize on the spectral sensitivity.
The Infrared films are films that have been more sensitized in the red
region and in the near infrared (around 800 nanometers).
Hot bodies are emitting infrared, but this is longer wavelenght infrared.
People are very often mistaken about that.
As far as I remember, a body should be very hot to emit near infrared, like
the sun for example, which is probably the infrared source of the majority
of IR-photographers.
I have a question too about infrared: Is it possible to find very fine
grain 35mm infrared films ? I guess not. I assume the crystals have to be
very large in order to trap the maximum number of IR photons ...?
Philippe Monnoyer