Also known as Common items. White items are of a basic level. Most items you find in the game that are not armor or weapons are white.
Many times quest items are White. They are important, but are not really rare in any way.
You will also find white armor and weapons in the game, these are bare bones, but still better than grey items. Some are quite good for the lowest levels. (1-10)
There are currently no white mobs in World of Warcraft. You may find NPCs with white names though.