var, let, and const are JavaScript keywords for variable declaration, differing mainly in scope, reassignment, and hoisting; var is function-scoped and hoisted with undefined, allowing re-declaration; let is block-scoped, hoisted without a value (Temporal Dead Zone), allows reassignment but not re-declaration; const is also block-scoped and hoisted without a value, but once assigned, its value (or reference for objects) cannot be changed. Modern JS recommends using const by default, let for reassignable variables, and avoiding var due to its confusing scoping.