  High-precision measurements by the ATLAS Collaboration are presented of
inclusive $W^+\to\ell^+\nu$, $W^-\to\ell^-\bar{\nu}$ and
$Z/\gamma^*\to\ell\ell$ ($\ell=e,\mu$) Drell-Yan production cross sections at
the LHC. The data were collected in proton-proton collisions at $\sqrt{s} = 7$
TeV with an integrated luminosity of 4.6 fb$^{-1}$. Differential $W^+$ and
$W^-$ cross sections are measured in a lepton pseudorapidity range
$|\eta_{\ell}| = 2.5$. Differential $Z/\gamma^*$ cross sections are measured as
a function of the absolute dilepton rapidity, for $|y_{\ell\ell}| < 3.6$, for
three intervals of dilepton mass, $m_{\ell\ell}$, extending from 46 to 150 GeV.
The integrated and differential electron- and muon-channel cross sections are
combined and compared to theoretical predictions using recent sets of parton
distribution functions. The data, together with the final inclusive $e^{\pm}p$
scattering cross-section data from H1 and ZEUS, are interpreted in a
next-to-next-to-leading-order QCD analysis, and a new set of parton
distribution functions, ATLAS-epWZ16, is obtained. The ratio of
strange-to-light sea-quark densities in the proton is determined more
accurately than in previous determinations based on collider data only, and is
established to be close to unity in the sensitivity range of the data. A new
measurement of the CKM matrix element $|V_{cs}|$ is also provided.
