Every object* that is above 0 Kelvin (read: everything in the universe) radiates somewhere on the electromagnetic spectrum. Stick with me, and I'll explain why. But first, every object* also absorbs light somewhere along the electromagnetic spectrum. For example, let's take glass. Now, in the visible spectrum (390<λ<700 nm), glass doesn't really absorb, reflect, or emit light (which is why it's transparent). This is due to a quantum mechanics phenomenon called "quantization." Essentially, electrons and the like can only exist in discrete levels of energy, and nowhere in between (like steps on a staircase—you can be on one step or another, but not between them (at least not without falling)). This is odd, and somewhat counter-intuitive because it doesn't appear to occur at the macroscopic level (our daily lives), but it's completely true (as far as we know). When an object absorbs light, it's because that specific wavelength of light is enough to jump an electron in it up an energy level (or more), because light is/carries energy. Similarly, when light is emitted by an object, it is of a particular energy level (or band) or the EM spectrum, because the electron can only jump down a discrete amount, releasing a set discrete amount of energy in the form of light (this is mostly how fluorescence works). So, in glass, visible light lacks the proper energy levels to jostle any electrons out of their cozy orbitals, and it just passes right through. In, say, a mirror, the electrons are excited by the light, but then jump right back down again by that same amount and emit the same wavelength back, causing reflection. In something like a lump of graphite, most wavelengths are absorbed.

That lump of graphite brings us to black bodies. A so-called "perfect" black body would absorb all wavelengths of light. No material with this property seems to exist in the universe, but that sort of thing has never stopped physicists from making theoretical conjectures about them. And these theoretical conjectures and observations can then be extended to "mostly black" bodies and applied to the real universe. So, if we had a perfect black body at room temperature (or any temperature above 0 Kelvin), it will still emit light. But how does it do that if it is absorbing all light? Well, temperature is a function of energy, and, by absorbing light, that black body is gaining energy and getting hotter (like a car sitting out in the sun). As it absorbs all light it encounters, it will rise in temperature and become hotter than the surrounding environment (and more energetic). The universe cannot abide an imbalance light that, so the black body needs to reach thermal equilibrium somehow (thermal equilibrium is the property that causes ice to melt in your drink but make it colder, or really any two differently-temperatured substances to strive to be the same temperature, by donating and excepting heat energy). In a vacuum, which is where all the best theoretical physics takes place, it must do this by emitting light (because, remember, from the paragraph above, emitting light makes you lose energy). In fact, the exact wavelength can be modeled using this wonderful formula Planck devised, which, as you can see, is entirely dependent upon the temperature of the body.

You can see black body radiation in everyday things, too, not just in our hazy theory-land. For example, look at the sun** (if it's nighttime out, grab some charcoal and light it, then wait until it's become awesome glowing embers). Ever wonder why it's so bright? Well, it's crazy hot—a giant ball of flaming gasses. But it's not so much the fire as the heat: because it's so hot (about 5,778K or 9,940°F on the surface—check out that temperature on the chart of Planck's law) it radiates light. The same applies to lava. Even though the rock is molten, logic dictates that it should be the same color: solid and liquid are merely states of matter that shouldn't change physical properties like color. It glows red because it's oh-so-very hot. It's where the expression red-hot and white-hot come from—the temperature of the substance actually dictates the wavelength of the light it will emit (which is why blue flame is hotter than red, and white is even hotter). It's a really neat property of matter that one doesn't think about too often, as we simply take heat-based radiation for granted. But there's some amazingly cool physics going on behind the scenes, as always.

* Except dark matter
** Fizzix Phriday does not advise that you look directly at the sun