The electromagnetic spectrum is divided into sections based upon wavelength. The visible part of the spectrum lies between about 380 and 750 nm. At slightly longer wavelengths, beyond what the human eye can see, is the infrared part of the spectrum between about 750 nm and 1 mm.
This wavelength range is dominated by thermal emission. In fact, the human body emits radiation primary at infrared wavelengths. As it turns out, dust in star forming regions glows brightly in the infrared. This dust absorbs ultraviolet radiation from stars, heats up, and then re-emits radiation at infrared wavelengths. By looking at these longer wavelengths, astronomers gain new insights into the structure and composition of the ISM. For example, let’s consider the constellation Orion. If you look in the optical you simply see the familiar stars (shown on the left below). But, in the infrared, the view is completely different. Each bright patch in the image on the right is a dusty cloud where young stars are forming. These young stars are completely invisible at optical wavelengths, but easily seen via their thermal emission in the infrared.
One unfortunate drawback about infrared astronomy is that the Earth’s atmosphere is mostly opaque at these wavelengths (most infrared radiation entering the atmosphere is absorbed by water molecules). So, astronomers are forced to turn to space. Space telescopes like Spitzer and Herschel have made great advances in infrared astronomy. Very recently, a new infrared telescope named SOFIA has been launched in a converted 747 airplane. By flying at altitudes of about 41,000 feet, SOFIA is able to get above much of the interfering water vapor in the atmosphere.