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What is threat ?
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Threat is a scalar value stored in what are called threat lists.
Each "normal" NPC (or server-controlled character) uses a threat list (though some behaves oddly).

Whenever you do something hostile toward a NPC, or if you get too close from an out-of-combat NPC,
or if you help someone the NPC doesn't like, you'll get added to this NPC's threat list.
Threat list contains all the possible PCs/NPCs a NPC finds nasty and could attack.

The NPC however, will not choose its target randomly (except a few exceptions);
it will instead choose the one which looks the most threatening.

That's where threat rolls in: it's a scalar value which is associated to any entity that is on a threat list.
Each time you deal damage, heal someone who is in a threat list etc., your threat value will increase
on one or several threat lists.

Threat management is important because it determinates where the damage will end up.
So you'll probably want the damage to end up to the tank: that's why there are all sorts of abilities/talents/effects etc.
that modify your threat level on one or several threat lists.

When you get the attention of a NPC, it's the act of getting "aggro". Aggro is also used as a verb.
The general rules of "aggro" are:
- You "aggro" an out-of-combat NPC by getting too close of it or debuffing/doing damage to it.
- You "aggro" a NPC already engaged in combat if you manage to enter on its threat list
and raise your threat value 30% above the guy who has currently aggro's threat value. % is diminished to 10% if you are at close range.

Also, as a side note, be aware that you'll stay locked in combat mode as long as you are in at least 1 threat list.
You'll only get out of it once you get out of every threat list that has you on it.

Hope this little speech made you understand better the mechanics of threat. :)

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What's generally a ThreatMeter ?
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A threat meter is an AddOn whose purpose is to determinate your threat level and compare it with your groupmates' one.

Such AddOns can use various ways to achieve this goal; until 2.4 most threat meters worked the following way:
each player running the AddOn calculates its own threat toward a given mob, and publish it through AddOn network to other party members, which
then receives the threat data and displays it into an organized table, which displays name, class and of course threat value of each player involved.

Now 2.4 has been released, let's see how this little world will evolve, as it is theorically now possible to avoid almost all data exchange
through network, and make each player independant threat-wise from the rest of the party :).
That's at least how I tried to build DTM, and I'm pretty satisfied of it. ;)