VectorQ[expr] gives True if expr is a list, none of whose elements are themselves lists, and gives False otherwise.
VectorQ[expr, test] gives True only if test yields True when applied to each of the elements in expr.
VectorQ[expr, NumberQ] tests whether expr is a vector of numbers.
See The Mathematica Book: Section 2.3.5 and Section 3.7.3.
See also: MatrixQ, TensorRank.