We present a comprehensive photometric and spectroscopic study of the nearby Type II supernova (SN) 2023ixf, with our extensive observations spanning the phases from ~3 to over 600 days after the first light. The aim of this study is to obtain key information on the explosion properties of SN 2023ixf and the nature of its progenitor. The observational properties of SN 2023ixf are compared with those of a representative sample of Type II/IIP/IIL SNe to investigate commonalities and diversities. We conduct a detailed analysis of temporal evolution of major spectral features observed throughout different phases of the SN 2023ixf explosion. Several key interpretations are addressed through a comparison between the data and the model spectra predicted by non-local thermodynamic equilibrium (non-LTE) radiative-transfer calculations for progenitor stars within a range of zero-age main sequence (ZAMS) masses. Our observations indicate that SN 2023ixf is a transitional SN that bridges the gap between Type IIP and IIL subclasses of H-rich SNe, characterized by a relatively short plateau (<~70d) in the light curve. It shows a rather prompt spectroscopic evolution toward the nebular phase; emission lines of Na, O, H, and Ca in nebular spectra all exhibit multipeak profiles, which might be attributed to bipolar distribution of the ejecta. In particular, the H{alpha} profile can be separated into two central peaked components (with a velocity of about 1500km/s) that is likely due to nickel-powered ejecta and two outer peak/box components (with a velocity extending up to ~8000km/s) that can arise from interaction of the outermost ejecta with a circumstellar shell at a distance of ~6.2x10^15^cm. The nebular-phase spectra of SN 2023ixf show good agreement with those predicted by a non-LTE radiative-transfer code for progenitor stars with a ZAMS mass ranging from 15 to 19 M_{sun}_. A distance D=6.35^+0.31^_-0.39_Mpc is estimated for M101 based on the expanding photosphere method.