Slug energy is a special type of energy that the Slugs give off.
Slug energy is a special kind of energy that allows life in Slugterra. It allows plants to grow and crystals to light. Different slugs give off different levels of Slug energy, for example, 2 containers of Fandango slug can resurrect an entire cavern easily, as they give off more energy then the common slug. It seems that all of The 99 Caverns are run by the energy of their slugs.
That Slug energy will go to work naturally on its own many of the Slugterrans have come up with many inevative ways to use Slug energy.
Slug energy appears to be the main power source for Slugterra. It powers all slug blasters and basic appliances. The Slug Terran Express has a sort of turbine similar to that of a Slug blaster meaning it probably runs on Slug energy or maybe ghoul energy.
All mecha beasts are powered by special hearts. The hearts are infused with Slug energy through molecular transfusion. This not only powers them, but also endows them with an almost partial sense of life.
In the episode Club Slug, Serge used an odd device in order to increase a Slug's power as part of his scam. It appeared to work by super charging a Slug's energy. It didn't last long however, and the effects quickly wore off causing the slugs to become tired and weak.
Ghouls give off there own energy but it is more destructive than Slug energy, being able to corrupt an entire cavern.
Ghoul energy, unlike Slug energy, seems to stunt the growth of plants but still allows the lighting of crystals.
Ghoul energy is used by less than honest characters for there own purposes.
Mecha beasts can be tainted by ghoul energy, in which case they start to go wild, their eyes also go red, and they become very aggressive. After a while they explode. Blakk tried to taint all the mecha beasts with an odd ghouling machine so everyone would have to use his Slug Terran Express.
Many of Blakk Industries machines glow an eerie red color like their blasters and trains may imply they run on ghoul energy.