OFDMA is an orthogonal frequency division multiple access scheme that uses OFDM and FDMA principles. It divides the available spectrum into multiple orthogonal subcarriers, and assigns subsets of subcarriers to individual users. This allows for efficient use of spectrum and multi-user diversity. OFDMA provides receiver simplicity compared to CDMA, but has some disadvantages like high peak-to-average power ratio and need for tight synchronization between users. It is used in wireless standards like WiMax due to its ability to support high data rates in frequency selective fading channels.