Nucleophilic substitution

A nucleophile is electron-rich — it has a lone pair or a region of negative charge looking for somewhere to go — and it attacks a carbon that is electron-poor, typically because a more electronegative atom nearby has pulled density away from it. Attaching to that carbon displaces whatever was there before, the leaving group, which departs taking the bonding electrons with it. That is the whole reaction in outline: electron-rich attacks electron-poor, something else leaves.

The mechanistic detail is where it gets interesting, because there are genuinely two distinct pathways. In one, the nucleophile arrives at the same moment the leaving group departs — a single concerted step, with no intermediate ever existing. In the other, the leaving group departs first, producing a short-lived carbocation intermediate, and the nucleophile attacks that afterwards, in a separate second step. These are not two names for the same thing: the two-step pathway is governed by how stable the intermediate carbocation is, so it is favoured at more substituted carbons where that intermediate is stabilised, while the one-step pathway is favoured at simpler, less hindered carbons and depends on how strong and available the nucleophile is.

Knowing which pathway is operating is what lets a chemist predict rate, stereochemistry and product identity correctly, rather than guessing. It is also worth separating nucleophile from base: both are electron-rich, and the same species — hydroxide, for instance — can act as either, but a nucleophile attacks carbon while a base removes a proton, and which role dominates in a given reaction depends on the substrate and conditions, not on some fixed property of the reagent alone.