What are electrophiles and nucleophiles?

An electrophile is a reagent that takes away an electron pair. In other words, an electron-seeking reagent is called an electrophile (E+). Electrophiles are electron-deficient and can receive an electron pair.

Carbocations $\left(\mathrm{CH}_{3} \mathrm{CH}_{2}^{+}\right)$and neutral molecules having functional groups such as carbonyl group  are examples of electrophiles.

A nulceophile is a reagent that brings an electron pair. In other words, a nucleus-seeking reagent is called a nulceophile (Nu:).

For example: $\mathrm{OH}^{-}, \mathrm{NC}^{-}$, carbanions $\left(\mathrm{R}_{3} \mathrm{C}^{-}\right)$, etc.

Neutral molecules such as $\mathrm{H}_{2} \mathrm{O}$ and ammonia also act as nulceophiles because of the presence of a lone pair.