Being colorless means that the card, effect, or whatever it refers to has no mana color association to it. Artifacts and lands, for example, are colorless (some artifacts in New Phyrexia and many in the Alara Block are colored). This can be either a deficiency or a bonus depending on how much either player's deck values colors. For example, Ghostly Flame was created to give black and red an answer to the Circles of Protection, since neither color has much of an answer to enchantments. It is also impossible to choose "colorless" as a color, so that makes it harder to defend against when something could affect all colors. However, you would probably not wish to use many colorless creatures when playing Earnest Fellowship, as they wouldn't have any colors to get protection from.
A nonland card does not have to be an artifact to be colorless, but there aren't many other cards that make themselves colorless. To date, the only nonartifact, nonland cards that are always colorless are Ghostfire and, by means of its static ability, Ghostflame Sliver, and Eldrazi, and Scion of Ugin. Red sometimes makes itself colorless in order to get around opposing protection from red and other such things. Many people mistake these cards for artifacts, but they are not. The picture fills in the sides and bottom.
| Colors of Mana