The difference between the different force fields can be found in many places, including the manual: Molecular Mechanics & Force Fields - Avogadro
Suffice to say if you can, use MMFF94 or GAFF for general organics and UFF otherwise.
The reason the energies will be different is because they’re relatively simple methods. Depending on what interactions are taken into account and the parameter, you will get different energies. Force fields don’t account for true quantum chemical interactions.