Why is light called em-radiation sometimes and thermal radiation other times? And sometimes people say that thermal radiation is another word for infrared radiation, but then other times thermal radiation is used to describe all em-radiation.

Also does an object with a lower temperature always emit light at a lower frequency? Just like really hot things seem to emit visible light. Do all extremely cold things emit radio wave and microwaves? What kind of light does a pillow or dry ice emit? And can something be so cold that it doesn't emit any em-radiation at all?

<i>Why is light called em-radiation sometimes and thermal radiation other times?</i>

Light is an electromagnetic phenomenon. It consists of oscillating electric and magnetic fields - hence em.

It is only called thermal radiation if it is produced by thermal processes (e.g. vibration of particles in a material).

<i>And sometimes people say that thermal radiation is another word for infrared radiation, but then other times thermal radiation is used to describe all em-radiation.</i>

Hot objects tend to emit light in the infrared region of the spectrum, so this is sometimes called thermal radiation. They might also emit visible light or even light of higher frequencies.

<i>Also does an object with a lower temperature always emit light at a lower frequency?</i>

All objects at a given temoerature emit a range of frequencies. The peak frequency - where most of the em radiation is emitted - varies with temperature. The colder the object the lower the peak frequency.

<i>Just like really hot things seem to emit visible light. Do all extremely cold things emit radio wave and microwaves?</i>

Yes, more or less.

<i>What kind of light does a pillow or dry ice emit?</i>

Mostly infrared, as you say.

<i>And can something be so cold that it doesn't emit any em-radiation at all?</i>

Not when you're talking about a macroscopic object. However, note that an object at the same temperature as its surroundings will be in thermal equilibrium - it absorbs as much light from its surroundings as it emits.

Well now, that was quick & simple. Thanx.