Like, the same stuff my characters trap in Soul Gems? Or is that just 'soul energy'?
That depends a bit on the game.
The mechanics of Morrowind and Oblivion treat them like batteries for enchanted weapons. The nuance being that Morrowind mechanics still embedded a bit of personality by noting who's soul it was and having weapons charge over time suggesting there was a soul at work. On the other hand, when enchanted weapons broke in Daggerfall the enchanting spirit would 'break' free and turn on it's owner.
http://www.imperial-library.info/obbooks/souls_black_white.shtml might also suggest that soulgems just capture a charge of energy as the soul can leak into a black soulgem. I think though that the admonishment isn't that different from one against sticking metal objects in a microwave.
However, http://www.imperial-library.info/mwbooks/feyfolken.shtml and the Redgaurd storyline twice over with -A'Tor and Iszara- requires that a soul holds that which makes a person. There are a few more but these are the most important.
So in conclusion, the mechanics have been sliding closer towards souls just being energy, but since they're just mechanics that change all the time we can just ignore them.