Glyphs are spell and ability cosmetic enhancements created by scribes. Until used, they are not soulbound and can be traded or sold. The colors used in glyphs are colors of the class for which they are meant. Glyphs are optional and not vital to game completion, although they may provide an edge.
Once learned, glyphs are permanent and may be switched out at any time outside of combat. Changing a glyph requires a [Vanishing Powder] for characters level 25-80, and a [Dust of Disappearance] for characters over level 80.
This section concerns content exclusive to Cataclysm.
Initially, there would be additional glyphs known as "ancient glyphs", added with the Path of the Titans system. When Path of the Titans was cancelled, the glyph system was changed to three tiers - Major, Medium and Minor. This was later changed to the system implemented in Patch 4.0.1 - Prime, Major and Minor.
Patch 7.0.3 (19-Jul-2016): The Glyph panel has been removed from the game. Some minor glyphs that had a cosmetic effect that does not apply to a specific spell have been converted into cosmetic items that provide the same effects as before.; Some minor glyphs that made cosmetic alterations to a specific spell have been added to the spell itself in the spellbook.
Patch 6.0.2 (14-October-2014): All characters now learn some important Major Glyphs automatically as they level up.
/Patch 4.0.1 (12-Oct-2010): Added Prime Glyphs and Glyph slots. The minimum level requirement to use glyphs is 25. You will unlock a Prime Glyph slot, a Major Glyph slot and a Minor Glyph slot at level 25, 50 and 75.