We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009. These data allow us to discuss the physical, excitation, and chemical properties of all the morphological components of the nebula, including its remarkable systems of knots and jets. The main results of this analysis are the following: (1) the electron temperature throughout the nebula is remarkably constant, Te[OIII]=10,200 K; (2) the bright inner rim and inner pair of knots have similar densities of Ne~6000 cm-3, whereas a much lower density of Ne~1500 cm-3 is derived for the outer knots as well as for the jets; (3) all the regions (rim, inner knots, jets, and outer knots) are mainly radiatively excited; and (4) there are no clear abundance changes across the nebula for He, O, Ne, or S. There is marginal evidence for an overabundance of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim have similar N/H values that are at variance with previous results. Our data are compared with the predictions of theoretical models, from which we conclude that the knots at the head of the jets are not matter accumulated during the jet expansion through the circumstellar medium; nor can their origin be explained by the proposed hydrodynamic or MHD interacting wind models for the formation of jets/ansae, since the densities, as well as the main excitation mechanisms of the knots, disagree with model predictions. Based on observations obtained at the 2.5 m Isaac Newton Telescope (INT) of the European Northern Observatory and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA for NASA under contract NAS5-26555.