This end result solution gives us a compound that is very stable, even under harsh circumstances, but when an activation energy is applied (usually in the form of a small shock-wave) the chemical compound very quickly degrades due to a high speed combustion, and causes an explosion, releasing a shock wave, a flash of light, and a ball of fire (heated gases).
This has a lot of applications, but how is TNT measured? As it turns out, TNT is such a reliable explosive, that the NIST (National Institute of Standards and Technology) actually defines the energy released by 1 gram of TNT to be 4184 Joules, or in shorthand, TNT releases 4184 J/g.
This makes things far easier than if we attempted to calculate things ourselves using calorimetry and a lot of other non-trivial means to do so, since there is a standard baseline, we now know what units to use.
Also, we need to define that 1 ton of TNT= 1 Mg of TNT, which allows us to perform comparisons, a many many explosives, despite what they use as reaction components, are measured in the blast yield as compared to TNT.
But let's have an example, to understand exactly what energy we're dealing with. 1 gram is about the mass of a paperclip. That is nothing near the kinds of values that we're going to be dealing with, so let's look at a little simple algebra with a test explosion that has been done in the past. Let's look at a recent test of "The Father of all Bombs".
The FOAB (father of all bombs) was a bomb detonated by the Russian air-force in 2007. It was an explosion that measured as approximately 44 tons of TNT. The picture on the right shows the initial explosion, just for visual effect.
Now, this bomb was 44 tons of TNT. Well, we know how much energy is in a gram of TNT, and we know how many grams are in a ton of TNT, so we can do a little bit of simple math, to figure out the energy released in Joules, which is a very convenient unit to use, because many things that have energies, can be described in terms of Joules.
1Mg TNT = 1x10^6 g TNT
1g TNT = 4184 J
1 ton TNT = 1 Mg TNT
So now we have our constants, we can do a little bit of math.
44 tons TNT= 44 Mg TNT
So we get that this massive non-nuclear explosion released around 184 GIGA JOULES of energy. that's a pretty large amount of energy to be thrown out of a comparatively small bomb. But, we have bigger craters to make, keep that number in mind, because we're coming back to it in a while.
http://hydra-media.cursecdn.com/minecraft.gamepedia
.com/1/1e/TNT.png?version=8ba6ad0a5b55e398a36ddb0326e87456
http://upload.wikimedia.org/wikipedia/en/4/49/Foab_blast.jpg
(The FOAB "Father of All Bombs")