What is a nucleophile and what is an electrophile?

A nucleophile is a chemical species that can donate a pair of electrons to a different chemical species (generally to an electrophile) to form a chemical bond in a reaction. They are generally negatively charged or are neutral with a lone pair of electrons avaliable for donation. Examples are H2O -OMe or -OtBu. Overall a nucleophile is electron rich.

An electrophile is a chemical species which is attracted to electrons i.e. negatively charged species (nucleophiles). Generally electrophiles are positively charged or they are neutral species with vacant orbitals that are attracted to an electron rich centre. In other words their electronic structure is incomplete, they are happy to accept and share electrons in their electron shells with elctron rich species. Exmples are H+ or alkenes. Overall an electrophile is electron defficient.