C4 is a very powerful and versatile explosive. It resembles modeling clay and can be molded into any shape, and is very stable. You can beat it with a hammer and it will just change shape, but will not explode. Soldiers light it on fire and use it to heat their rations.
To detonate C4, a blasting cap is inserted into the C4, a wire goes from the blasting cap to an electrical source, and when the current is turned on the blasting cap explodes, causing the mass of C4 to explode at a rate of 26,000 feet per second.
In The Templar Concordat, this is the explosive used in the bombing.