Matter IS energy, and energy IS matter. I like to think of it as a phase transition, personally, but I don't know exactly HOW it happens.

Real life applications: Nuclear power works because the mass of the products in a nuclear reaction is less than the mass of the reactants. The mass that is "lost" in these reactions is converted into energy.

The reason we use fission reactors is that it can be done at easily-attainable temperatures. Fusion, on the other hand, requires very high temperatures, such as those in the sun. We can replicate those temperatures using arrays of lasers that are all focused on the same point, and then nuclear fusion can take place. Unfortunately, at this point I don't think we've yielded a net gain in energy through fusion, so fusion reactors aren't feasible yet. In the future though, who knows?

the big crunch

Einstein motherfucker.

Matter is just the conclusion humans have come to in order to explain the existence of objects and material. Energy is everything, matter is just one manifestation of energy.

The best comparison I can think of is water. It exists as gas, solid, and liquid. Think of energy as water overall and maybe matter as one of the three phases.

Matter is made up of atoms. Atoms are made up of a nucleus and electrons, both of which are there and not there. When you need something made up of matter, such as a can of pop, energy manifests itself in the form of a can of pop.

Energy > Matter > Can

Matter is just a middleman to explain the physical presence of things in an individual's reality.

Nukes work by making subatomic particles go near the speed of light. Once that happens, they turn into energy.