Glyphs are items made by the Inscription profession. Each glyph modifies one of your spells, changing it slightly. At 80, you can have 3 Major Glyphs and 3 Minor Glyphs. Major Glyphs are powerful, while Minor Glyphs tend to be cosmetic.
Glyphs need to be applied near a Lexicon of Power. This can be found in the major cities near the Inscription trainer. Essentially, you have to return to a city to change Glyphs, so you can't change them in the field. However, I think they will rapidly become fairly cheap, so re-glyphing might be something you do before you head out to an instance for the night.
Here's a listing of what I think the top 3 major glyphs are for Holy and Protection at level 80, and some comments on the others. I'll cover Retribution and Minor glyphs in another post.
1. Glyph of Seal of Light - You need a Seal up to get the haste from Judgements of the Pure. This improves your healing by 5%, making it very powerful, especially on shorter fights.
2. Glyph of Seal of Wisdom - Like Light above, but for longer fights. It's an interesting decision if you should take both of the Seal glyphs, because you can only use one at a time. But it makes you a bit more flexible for different fights.
3. Glyph of Holy Light - This can do a fair amount of splash healing, helping a traditional paladin weakness.
Glyph of Cleansing - Mana reduction is always useful, but not very exciting.
Glyph of Divinity - Lay on Hands doesn't drain your mana anymore, and only has 20 min cooldown. So it's a very useful spell, and this glyph makes it even more useful. You get to drop a large instant emergency heal, and you get a fair bit of mana back.
Glyph of Flash of Light - This glyph is currently terrible. The HoT does not stack, so you severely hurt your tank-healing capabilities. It might be decent if you were raid-healing, but given that Glyphs are permanent, it's really hard to see this being useful. Perhaps a Ret build (with Art of War) or a levelling build would consider it, but it's pretty much a waste otherwise.
Glyph of Seal of Righteousness - Might be good for soloing, but in an instance you will be using a glyphed Light or Wisdom.
1. Glyph of Righteous Defense - Having taunts miss usually ends very badly.
2. Glyph of Seal of Vengeance - Vengeance is a solid seal for Protection, and reducing boss dodge and parry rates by 2.5% is very nice.
3. Glyph of Avenger's Shield - This glyph is a bit of a toss-up. I think it will become more useful as you move into raiding, and precise pulling and having large initial threat becomes more important. However, you might prefer building threat on multiple mobs in the 5-man dungeons. As well, 100% more damage makes it more feasible to toss AS on every cooldown, just to build threat.
Glyph of Consecration - This glyph does increase damage per cast of Consecration. However, Protection has a very smooth rotation with an 8-9s Consecration cooldown, and this glyph spoils that rotation. Still, if you're very comfortable with managing your abilities as they come off cooldowns, this might be a good choice for you.
Glyph of Exorcism - Gives you another spell interrupt, which is something we lack. However, this is very specific and may depend greatly on what instance you are planning to run.
Glyph of Judgement - Extra damage and threat from one of your main abilities is always good.
Glyph of Seal of Righteousness - I think most Protection paladins will be using a glyphed Vengeance on bosses, but this could be decent for trash.
Glyph of Spiritual Attunement - You shouldn't need this glyph, as regular SA and Blessing of Sanctuary will provide enough mana. But keep it in mind if you do find you have mana problems.