With Wrath of the Lich King arrives glyphs - follow this guide to make sure your Death Knight is using the best glyph for you.
Glyphs are created by the Inscription profession and enhance specific spells and abilities. As such, they are class specific so when searching on the Auction House or browsing an Inscription profession linked list from a player you can look under the Death Knight heading to see all those relevant to you.
There are two types of glyphs, major and minor. Major glyphs will generally be of a more active nature and will help you in combat. Minor glyphs are usually cosmetic or logistical in nature, though the Death Knight minor glphys are unusual in that they actually affect combat performance.
Glyph slots open up based upon your level
Level 15: 1 major 1 minor
Level 30: 2 major 1 minor
Level 50: 2 major 2 minor
Level 70: 2 major 3 minor
Level 80: 3 major 3 minor
Death Knights, starting at level 55, will begin with 2 major and 2 minor glyph slots open for use.