I've read that it is also a Lie group. But what does that have anything to do with special relativity or different reference frames? The wiki definition is "a 10-dimensional noncompact Lie group. The abelian group of translations is a normal subgroup while the Lorentz group is a subgroup, the stabilizer of a point. That is, the full Poincaré group is the affine group of the Lorentz group, i.e. the Poincaré group is a semidirect product of the translations and the Lorentz transformations:" ...blahblahblah Can anyone please cut the bs out for me and tell me in layman terms the gut of what it is?