well to those that play those genres (i don't, personally) or listen intensively (i do!), there's a pretty big difference. Obviously we're still talking death metal so there will be overlap between genres, but there's a pretty big difference between bands like Atheist and Cynic (tech death) and Cannibal Corpse and Devourment (brutal). Some people might argue that brutal death is an umbrella term for all non-melodic death metal (melodic death metal meaning bands like In Flames, Dark Tranquility, etc), but these days its seemed to become a more specific sub-genre. Ah, the infinite complexities of metal, haha. I know its all lost on the non-fans out there, so apologies if i'm boring you. Think of it like the differences between bebop and acid jazz, if that helps.
I was just trollin. I'm in a melodic death metal band myself
