What is inscription?
Inscription is the new profession that will be added with Wotlk, like JC was the new profession for TBC. The profession is available to everyone, even is DK's get a bonus, think it was +15 skill. Inscriptionist's will be able to make Glyphs to enhance abilities of yourself and other players, inscriptionists will also be able to make some consumables.

What is a Glyph?
In Wrath, characters will have an extra page in their spellbook/character sheet.(Screen will be added later tonight when i can get home and upload it).Basically it boils down to 6 glyph slots for character improvements. The slots are broken into the following categories, Greater Glyphs, Lesser Glyphs and Minor Glyphs. As you see on the picture, there is room for two Greater-, two Lesser- and two Minor Glyphs.

Greater Glyphs will enhance class spells and abilities greatly, here is an exampel of a known glyph which is most likely a greater glyph:

Venomous Mana (Glyph Passive) - Empowers a Greater Glyph to grant your Viper Sting ability an additional 15% chance to resist dispel effects.

Lesser Glyphs will offer smaller buffs to class abilities, you can expect lesser glyphs to have a similar strength to:

Glyph of Moonfire (Class: Druid) - Empowers a Lesser Glyph to reduce the rage cost of your Demoralizing Roar by 20

Minor Glyphs will offer cosmetic buffs or slight buffs to some spell mechanics or cost's a couple of examples of minor glyphs beign used for ciosmetic buffs would be:

Glyph of Polymorph - Sheep spell to turn the target into a baby penguin.
Glyph of the White Bear - Changes the appearance of your bear and dire bear forms to that of a polar bear.

You mentioned Consumables?
Yes. Inscriptionists will be able to make their own range of consumables same as many other professions. It can be it runes of warding from blacksmithing or wizard oil from enchanting.

Notes
In Wotlk, Herbalism is the primary gathering profession for Inscription. Inscriptions require herbs as materials, and so it relies on the herbalism gathering profession, Glyphs can also be sold at the Auction House. Those who don't purchase the expansion will be able to use items created by an inscriber, but only those players who purchase Wrath of the Lich King will be able to learn Inscription as a profession.

A skill called Milling will accompany Inscription that will produce reagents from herbs similar to how the Prospecting skill work of ores.