Difference Between Mesomeric and Electrometric Effect

in the presence of an electophile if there is a transfer of pi electrons from 1 atom to the other to produce temporary polarity is called electromeric efffect. here when the electrophile is removed the charge disappears and the subtrate attains its original form but when the transfer of pi electrons takes place consecutively giving a permanent polarity it is called mesomeric effect

to keep it simple...
mesomeric effect is a permanent polarity in a molecule due to the transfer of electrons from pi orbitals within the molecule. it is also called as resonance effect.

but in case of electromeric effect a charged particle (mostly +ve) will be involved. the transfer of the electron will facilitate this electrophile to bond with the molecule. this electrophile is considered as the attacking reagent.