The shaped charge is a detonation pack generally only found in warfare maps. It is usually used to destroy barricades or trigger special events, such as the avalanche in the avalanche map.
Once picked up, it remains in your hands as your only weapon, and you are not allowed to switch to another weapon until you use it.
To use it, press the primary fire button and it will be dropped and set. There is a short timer (5 seconds?) and then it will detonate, causing massive damage in a small area.