A family is a parametric component definition - a door, a column, a light fixture - with types (predefined size and property combinations) and instances (the actual placed elements). Revit has three kinds: system families (walls, floors, roofs - built into the project), loadable families (.rfa files you create or download), and in-place families (one-off elements modeled directly in the project). Mastering families is the single biggest step from "using Revit" to "controlling Revit".