Insights into nucleon structure from parton distributions

We review recent progress in understanding the substructure of the nucleon from global QCD analysis of parton distribution functions (PDFs). New high-precision data on $W$-boson production in $p\bar p$ collisions have significantly reduced the uncertainty on the $d/u$ PDF ratio at large values of $x$, indirectly constraining models of the medium modification of bound nucleons. Drell-Yan data from $pp$ and $pd$ scattering reveal new information on the $\bar d-\bar u$ asymmetry, clarifying the role of chiral symmetry breaking in the nucleon. In the strange sector, a new chiral SU(3) analysis finds a valence-like component of the strange-quark PDF, giving rise to a nontrivial $s-\bar s$ asymmetry at moderate $x$ values. We also review recent analyses of charm in the nucleon, which have found conflicting indications of the size of the nonperturbative charm component.