Interpreting virtual photon interactions in terms of parton distribution functions

Interactions of virtual photons are analyzed in terms of photon structure. It is argued that the concept of parton distribution functions is phenomenologically very useful even for virtual photons involved in hard collisions. This claim is illustrated on leading order expressions for $F_2^{\gamma }{(x,P}^2{,Q}^2)$ and the effective parton distribution function $D_{\mathrm{eff}}{(x,P}^2{,Q}^2)$ relevant for jet production in ${\gamma }^{*}p$ collisions, as well as within the next-to-leading order QCD calculations of jet cross sections in $\mathrm{ep}$ collisions.