Mercantilism is less a school than the operating assumption of European statecraft from the sixteenth century to the eighteenth, and it is named chiefly by its enemies — Adam Smith coined "the mercantile system" in order to demolish it.
Its core claim is that a nation's wealth consists in its stock of treasure, and that the object of policy is therefore a favorable balance of trade: export more than you import and the difference arrives as bullion. From this follow the instruments that defined the period — tariffs, navigation acts, chartered monopolies, bounties on export, prohibitions on the export of raw wool or skilled artisans, and colonies understood as captive markets and sources of materials that must not be allowed to manufacture for themselves.
Underlying it is a picture of trade as zero-sum. Thomas Mun, writing for the East India Company, argued that a nation grows rich exactly as its neighbors grow poor; Colbert built French industrial policy on the premise and made the state the principal entrepreneur. The doctrine was never systematized by its holders because it did not need to be: it was what ministers and merchants took for granted.
Its importance to everything downstream is that it supplied the target. The Physiocrats attacked it for mistaking treasure for wealth and neglecting agriculture; Smith attacked it for confusing money with the goods money buys, and for legislating in the interest of producers against consumers. Free-trade liberalism is unintelligible except as an argument against this system, and every subsequent revival of protection — List's national economy, twentieth-century import substitution, contemporary industrial policy — is a return to some part of it.