Matter and energy can be considered interchangable. Matter being congealed energy...semantics plays a big part in physics.
When uranium 235 is fissioned, there is conversion of matter into energy. The fission fragments and neutrons are slightly less massive than the original uranium atom. The energy released is in accordance with Einstein's famous equation E=mc^2...where m is the change in mass from the uranium to the fission fragments. With fusion, hydrogen combines to form helium as in the sun. Again, the helium is less massive than the original hydrogen atoms with the resulting release of energy. The energy released in a nuclear reaction is about one million times greater that any chemical reaction.
Now, converting energy into matter...that is a bit tougher. In general, we really can't convert energy into matter easily, so Scotty isn't going to be beaming anyone anywhere in the future. There are reactions that can convert energy to matter, e.g. If a high energy gamma ray smacks a heavy nucleas, it can form an electron and a positron. The energy of the gamma must be suffiecient in accordance with Enstein's formula again to convert the energy into two massed particles. If matter just absorbs the energy of photon, then, in general it will just heat up--the atoms will wiggle faster.
Energy=matter=energy.
glenn