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.
Major glyphs feature spell changes that may increase the damage, or another feature of the spell. Some of these glyphs have trade-offs. For example increasing direct damage but removing a DoT effect of the spell.
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 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.