  We give a detailed account of the recently formulated generalized vector
dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low
$x\cong Q^2/W^2$, including photoproduction. The approach, based on $\gamma^*(q
\bar q)$ transitions, $q \bar q$ propagation and diffractive $(q \bar q)p$
scattering via the generic structure of the two-gluon exchange, provides a
unique and quantitatively successful theory for the $\gamma^* p$ total cross
section, $\sigma_{\gamma^* p} (W^2,Q^2)$, at low $x$. The GVD/CDP is shown to
imply the empirical low-$x$ scaling law, $\sigma_{\gamma^* p}
(W^2,Q^2)=\sigma_{\gamma^* p} (\eta)$ with $\eta=(Q^2+m_0^2)/\Lambda^2(W^2)$,
that was established by a model-independent analysis of the experimental data.
