Rom hacking is taking a preexisting (for Pokemon usually GBA games) and modify by adding or removing features within the preset limit of the original cartridge (for GBA would be 16bit), typically using editing programs made by the hacking community
RPG Maker (with essentials) is closer to making a game with less limitations as it dose not have to obey a 16bit limit rule and you do not have to rely on external editors (while are are a few, you can still do everything from inside essentials), but requires a lot more work as you do not have a complete game to work with, you must create your own maps, NPCs, events ect... Luckily those that run the project like Maruno have already made example maps, events and much more to give a guide line of how to setup similar events. Also since you are not depending on external editors it is recommended to learn RGSS (Ruby Game Scripting System, a.k.a Ruby) for more information check out the (broken link removed)
TL:DR
Rom Hack = take pre made game and tweak it within a given limit.
RMXP/Essentials = have to make most things yourself, but the only limits are your imagination and willingness to learn.
