For Java, the rank hierarchy is like this:
Stone (Default) -> Iron -> Lapiz -> Gold -> Diamond -> Emerald -> Obsidian (Top Donator Rank) (-> Plus, monthly subscription rank)
On Java, rank perks include cosmetics, voting in games (for different features, per-game), name colour, chat colour, double-jump/flying in lobbies, larger friends list/party size and extra Cubelets (cosmetic chests).
On Bedrock, it's a bit different. You buy packs, egg skins, etc, and you get rewarded a number of VIP Experience Points. This contributes to increasing your VIP Rank, from VIP 1 to VIP 5, each requiring a larger amount of VIP Experience to attain.
On Bedrock, rank perks include name colour, profile banner, cosmetics, larger friends list/party size and a 'buddy' (not sure what this is, perhaps it follows you around in the lobby?).
Hope this helps!