Nice work SculptedCold>>
I'll give my 2 cents - the simple version......
grind = death metal + punk - simple but heavy riffs, raw production, short songs, generally fast-paced, lots of blasting and catchy mid-tempos, vocals can be growling, shouting, screaming or a combination of all three. Lyrical themes range from political to funny to anything really. eg. Rot, Anal Cunt, Excruciating Terror, Birdflesh, Dahmer
goregrind = pretty much the same as grind but often with a more dirty/muddy sound, pitch-shifted gurgly vocals, often slower and even more simple in structure than grind, lyrics and themes are all gore-oriented. Horror movie samples often incorporated in songs or used for intros. eg. Neuro-Visceral Exhumation, Last Days of Humanity, Dead Infection, Squash Bowels
porngrind = pretty much the same as goregrind but with everything porn-themed. Porn samples, porn lyrics, porn artwork, you get the idea. Really good porngrind bands (IMO) are able to capture a really sleazy, dirty feeling to their music. eg. Gut, Purulent Spermcanal, Creamface, Mucupurulent, Rompeprop
then there's the newer "tech-grind" bands like Pig Destroyer, AgNb, Dillinger Escape Plan, Discordance Axis who are a combination of grindcore and tech death metal I spose.
ok I'm done now.