All commercial infrared films are sensitive to a combination of visible and infrared light. When treated like normal film (normal speeds, no IR filter) they behave like a normal film; they show no special characteristics.
"Infrared pictures without filters always produce results similar to black and white pictures, missing [the IR] tonal value graduation. Color IR pictures show a strong blue-green tint and little color differentiation."
Therefore, you must block out visible light in order to get prints with the characteristics associated with infrared photography like white foliage, dark sky (etc.) This is achieved with filters.
Camera lenses are normally calculated for the visible spectrum. Focusing scales of high-quality lenses have an infrared index (a red dot next to the focus markings) to which the focus is set for infrared photography.
IR Film Correctly (ISO ratings & other factors) Some IR films can be shot at various ISO settings and others can not. Changing the ISO can produce interesting effects. For example, According to Laurie White Hayball, rating Kodak at ISO 100 increases grain and lowers contrast, ISO 400 increases contrast and lessens grain. There is also ISO flexibility in Ilford SFX 200 film. It can be rated from ISO 50 to 200. Not all films have such flexibility. For example, Konica 750 does not underexpose well. It is best to test a film's abilities beforehand.
There is also filter factor to take into account. According to B+W, the average sensitivity of black and white infrared film is about ISO 50/18-degrees. Kodak infrared film with the B+W filter #092 is about ISO 20/14-degrees; with the B+W filter #093 it is about ISO 10/11-degrees.
Thus, precise ISO ratings are difficult to obtain. It is advisable to first test a film and filter combination.
Infrared films are responsive to all visible light and some ultraviolet light. Visible light starts at about 380nm and gives way to invisible IR light at around 730nm.
As mentioned earlier, if you do not block out the visible light it will overpower the IR effect leaving you with a regular-looking negative. That is why a yellow, orange, red or opaque filter is a must in IR photography. Different filters block out different amounts of visible light.
*Orange-colored filter reduces the blue-green sensitivity for color infrared films. It helps avoid blue tint and differentiates colors more evenly. Filter factor depends on film sensitivity.
If you really want to perfect your infrared photography, these two books by Laurie White are fantastic. Within their clearly-written pages you will find a wealth of information about shooting infrared in easy-to-understand language. She thoroughly covers everything you need to shoot infrared including films, filters, printing and more.