The catastrophe model of the solar eruption predicted that a long current sheet (CS) develops in the major eruption. The CS length and the dynamics of the ejecta are dominated by both the rate of magnetic reconnection and the background magnetic field. Formation and development of the long CS were confirmed by observations soon after it was predicted. Furthermore, the CS was found much thicker than expected by the traditional theory. Works in both theories and observations regarding the physics behind the unexpectedly thick CS have been performed for many years, but the question is still open. Recent numerical experiments suggest that the turbulence due to various instability inside CS accounts for the thick CS and fast reconnection that drives the major eruption. In this review, we first introduce the model and its prediction of the long CS, and the corresponding confirmations by observations. Then, we discuss studies of various CS features performed so far, including investigations of its geometric scale and internal fine structures, together with the possible mechanisms for observations. Finally, we list the differences between the classical understanding of the CS and the facts revealed by both the observations and numerical simulations, and discuss possible reasons why the differences occur.