For any truncated path algebra Λ of a quiver, we classify, by way of representation-theoretic invariants, the irreducible components of the parametrizing varieties Rep d (Λ) of the Λ-modules with fixed dimension vector d. In this situation, the components of Rep d (Λ) are always among the closures Rep S, where S traces the semisimple sequences with dimension vector d, and hence the key to the classification problem lies in a characterization of these closures.Our first result concerning closures actually addresses arbitrary basic finite dimensional algebras over an algebraically closed field. In the general case, it corners the closures Rep S by means of module filtrations "governed by S"; in case Λ is truncated, it pins down the Rep S completely.The analysis of the varieties Rep S leads to a novel upper semicontinuous module invariant which provides an effective tool towards the detection of components of Rep d (Λ) in general. It detects all components when Λ is truncated.