What is the resonance effect? Write about its types. Or,what is the positive and negative resonance effect? Explain with examples.

(N/A) The resonance effect is defined as the polarity produced in a molecule by the interaction of two $\pi$-bonds or between a $\pi$-bond and a lone pair of electrons present on an adjacent atom. The effect is transmitted through the conjugated system. There are two types of resonance or mesomeric effect,designated as the $R$ or $M$ effect.
$(a)$ Positive resonance $(+R)$ or mesomeric $(+M)$ effect:
$(i)$ Definition: When the transfer of electrons is away from an atom or substituent group attached to the conjugated system,this electron displacement increases electron density at certain positions in the molecule. This is called the positive resonance $(+R)$ effect.
$(ii)$ Example: In aniline,the $-NH_2$ group exhibits a $(+R)$ or $(+M)$ effect. The lone pair of electrons on the nitrogen atom is transferred into the benzene ring,making the molecule polar and increasing electron density at the ortho and para positions.
$(iii)$ Other examples of groups exhibiting $(+R)$ or $(+M)$ effects include: $-X, -OH, -OR, -OCOR, -NH_2, -NHR, -NR_2, -NHCOR$.
$(b)$ Negative resonance effect $(-R)$ or negative mesomeric effect $(-M)$:
$(i)$ Definition: When the transfer of electrons is towards the atom or substituent group attached to the conjugated system,these groups are said to exert a $(-R)$ or $(-M)$ effect.
$(ii)$ Example: In nitrobenzene,the $-NO_2$ group exhibits a $(-R)$ or $(-M)$ effect. The $\pi$-electrons of the ring are shifted towards the nitrogen atom of the $-NO_2$ group,causing the ring to become polar and decreasing electron density at certain positions,making them electron-deficient (positive).