  We study geometrical properties of an Ulrich vector bundle $E$ of rank $r$ on
a smooth $n$-dimensional variety $X \subseteq \mathbb P^N$. We characterize
ampleness of $E$ and of $\det E$ in terms of the restriction to lines contained
in $X$. We prove that all fibers of the map $\Phi_E :X \to {\mathbb G}(r-1,
\mathbb PH^0(E))$ are linear spaces, as well as the projection on $X$ of all
fibers of the map $\varphi_E : \mathbb P(E) \to \mathbb P H^0(E)$. Then we get
a number of consequences: a characterization of bigness of $E$ and of $\det E$
in terms of the maps $\Phi_E$ and $\varphi_E$; when $\det E$ is big and $E$ is
not big there are infinitely many linear spaces in $X$ through any point of
$X$; when $\det E$ is not big, the fibers of $\Phi_E$ and $\varphi_E$ have the
same dimension; a classification of Ulrich vector bundles whose determinant has
numerical dimension at most $\frac{n}{2}$; a classification of Ulrich vector
bundles with $\det E$ of numerical dimension at most $k$ on a linear $\mathbb
P^k$-bundle.
