In this paper, we analyze the symbol error rate (SER) performance of decode-and-forward (DF) cooperative communications over Nakagami-m fading channels. For the classical three-node cooperation model, the exact SER expressions for both M phase shift keying (PSK) and M quadrature amplitude modulation (QAM) are derived. For easy usage, we also provide asymptotic approximations for the SER. In the analysis, we first consider the cooperation scenario in which all the channels are uncorrelated. In practice, it may happen that a correlation exists between the source-destination and relay-destination channels, e.g., for user cooperation in the uplink of a cellular network. We then examine the SER in this scenario. In addition, by varying the parameters of the Nakagami-m fading channels in the analytical results, we obtain some perceptions of choosing a good relay for enhancing the cooperation and make clear about how the included Nakagami fading figures and channel variances affect the SER performance. In addition, the optimal power allocation is investigated based on the derived asymptotic SER expressions. Simulations are finally provided to verify the correctness of our analysis.