Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic ep scattering regimes have been measured with the ZEUS detector at HERA using an integrated luminosity of 82.2 pb(-1). The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies E (jet)/(T) > 17 GeV. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quarkjet initiated jets. Jet cross sections as functions of E (jet)/(T) , jet pseudorapidity, the jet-jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of alpha(s) (M-Z) of alpha(s) (M-Z) = 0.1176 +/- 0.0009(stat.) (+0.0009)/(-0.0026) (exp.) (+0.0091)/(-0.0072)(th.) was extracted from the measurements of jet shapes in deep inelastic scattering. (C) 2004 Published by Elsevier B.V.