Inscription is a new profession which was introduced in Wrath of the Lich King with which players can create Glyphs which are enhancements to an individual's spells or abilities, Scribes can also make Scrolls and items you can equip in your off-hand.
Inscription use herbs as main materials. When you learn Inscription, you will get a skill called Milling, which you can use to produce pigments from herbs similar like prospecting for Jewelcrafters. You will need Pigments to make Inks, Inks are needed to create Glyphs.
Glyphs are special items that can be used to enhance a specific spell or ability. They are usually just visual changes to your spells, none of them will make your character stronger. For example a Death Kinght's [Raise Dead] spell summons a geist instead of a ghoul.
Glyphs are consumed when you use then, and if you decide you want to remove a Glyph from a spell, you can use [Vanishing Powder]
How to apply a Glyph to your spell?
You just simply have to right click the Glyph in your inventory, then it will automatically open your spellbook and highlight the spell that you need to apply the Glyph.
If you learned everything you wanted to know, visit my Inscription leveling guide and start leveling Inscription.