Hadron Physics at RHIC

From the first five years of the relativistic heavy ion runs at RHIC, several key experimental discoveries brought a new understanding of the matter at the high temperature and energy density. Among the rich data from four experiments (BRAHMS, PHENIX, PHOBOS, and STAR), the following results are important to characterize the matter created in central Au+Au collisions at RHIC: (1) large hadron yield suppression at high $p_T$, (2) disappearance (or broadening) of back-to-back jet, (3) ``baryon anomaly'' at intermediate $p_T$, and (4) large event anisotropy and its quark number scaling behavior. We will review those experimental results along with the context of a possible formation of the strongly interacting quark-gluon plasma. We will also present the most recent systematic studies on hadron production at RHIC, such as single particle spectra and elliptic flow in different beam energies and colliding systems (p+p, d+ Au, Cu+Cu), which may give us a further understanding of the in- medium effects and their evolutions.