Gray

The color gray is used in World of Warcraft in various different ways. One way is as part of the usual Blizzard Entertainment quality color system for items (gray, white, green, blue, purple, orange, red) or difficulty for mobs (gray, green, yellow, orange, red) and tradeskill recipes (gray, green, yellow, orange).

Gray Items
Gray items are often referred to as Vendor Trash. These are items that generally have no good use compared to other items you could get that are of higher quality.

Gray items, despite their uselessness, can sometimes sell for a good amount to a vendor.

Some gray items have also recently been given uses at the Darkmoon Faire for ticket turn-ins (these items have since been upgraded to white status in recognition of their enhanced role).

Gray Mobs
Gray mobs are of a level much lower than yours. These mobs do not give experience. Once a mob is grey to you, it is normally quite easy to kill. There are some exceptions though.

Warlocks can not gain Soul Shards from Gray mobs.

Also, if, when the mob in question is targeted, and the plate at the top of the screen indicates its name with a yellow name on a gray background, it might be said to be gray. This means that someone else will be credited when that mob is killed.

Gray Levels (PvP)
In PvP, when someone's level is gray to you, you cannot gain any Honor points from them. So, in other words, it is not worth your time to kill them.

Gray Recipes
In your profession window, a recipe shown in gray will never give you a skillup when used. These recipes are considered "easy".

Gray Nodes
For gathering-type professions (skinning, herbalism, mining, also rogue lockpicking), a gray node (skinnable corpse, etc.) will not give you a skillup when used.

Gray Quests
In your quest window, a quest shown in gray is intended to be completed at a much lower level than your current level. You will receive little, if any, experience for completing gray quests.