Complex sentence: Sphenacodontidae is the group containing "Dimetrodon" and several other sail-backed synapsids like "Sphenacodon" and "Secodontosaurus", while Therapsida includes mammals and their mostly Permian and Triassic relatives.
Split into simpler sentences: Sphenacodontidae is the group containing "Dimetrodon" and several other sail-backed synapsids like "Sphenacodon" and "Secodontosaurus". Sphenacodontoidea contains Sphenacodontidae and Therapsida, a clade that includes mammals and their mostly Permian and Triassic relatives.