Although they aren't scientific definitions, they are somewhat helpful. In general, sps like acropora (e.g., staghorn) or montipora need more intense lighting and higher water quality than lps, and sps aren't as aggressive. LPS, like hammer coral or bubble coral are easier to keep, but can send sweeper tentacles to attack nearby corals. Now I fully expect to be flamed for the above overgeneralizations, and one should always be careful to learn about the specific needs and bad habits of a coral one wants to keep. They are all photosynthetic, by the way.