The warmth or coolness of light is known as its color temperature. This has nothing to do with actual heat from a light bulb, but only with the appearance of the light. Besides using terms like "warm white," "cool white," and "daylight" to describe these color temperatures, we also rate these in kelvin, which is a temperature scale.
It's important to note that there is no absolute consensus on what kelvin ratings equate to what color temperature descriptions. Visit different websites and you'll get different answers. But the following is a good general guideline to what these descriptions mean. (Also see the image at the end of this post.)
A "warm white" bulb is usually considered to be below 3000K (3000 kelvin). This is the light color provided by an incandescent bulb (2700K) or halogen bulb (2850K). As an incandescent or halogen bulb is dimmed, it becomes even warmer -- slightly orange -- in appearance. It may dip to about 2200K or below. The lower the kelvin, the warmer a bulb appears.
CFLs and LEDs can also provide a "warm white" appearance, and you'll need to choose CFLs or LEDs if you want the other color temperatures listed below.
Bulbs that provide light at around 3000K to 3500K may be considered "white" or "soft white."
Bulbs that provide light at around 4100K to 5000K are considered "cool white" and these start to have a slightly blue feel to them.
Bulbs that provide light at around 6500K are considered "daylight bulbs" and these have a definite blue and cool sensation to them.
Take a look at the image below and you'll get an idea of how different lighting can affect an office setting. Choosing between these types of light bulbs is a personal decision. In a household setting, you may like one color temperature and stick with that throughout the home; or you may choose different color temperatures to set a different feel for each room. For instance, you may want warm bulbs in dining rooms and bedrooms, white bulbs in kitchens, and cool bulbs in utility areas like laundry rooms and workshops. Again, it's a matter of personal taste.
When you shop for light bulbs at Lighting Supply, you'll find a refinement option on our site to sort bulbs according to their color temperature to help you more easily find the bulbs you need.