KLOE has measured most decay branching ratios of K_S, K_L amd K^+- mesons. It has also measured the K_L and the K^+- lifetime and determined the shape of the form factors involved in kaon semileptonic decays. We present a description of the above measurements and a well organized compendium of all of our data, with particular attention to correlations. These data provide the basis for the determination of the CKM parameter V_us and a test of the unitary of the quark flavor mixing matrix. New interest for a measurement of the ratio R_K = Gamma(Ke2)/Gamma(Kmu2) has recently arisen. Since the electronic channel is helicity-suppressed by the V-A structure of the charged weak current, R_K can receive contributions from physics beyond the SM. We present a new measurement of the ratio R_K with about 1% accuracy based on ~2 fb-1 of integrated luminosity. We describe the theoretical interpretation of the above tests and set bounds on new physics.