JPL's Wireless Communication Reference WebsiteChapter:
Analog and Digital Transmission

In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. OFDM is significantly less sensitive to intersymbol interference, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequencytime window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequencynonselective channels. Applications of OFDM are found in
MultiCarrier Modulation on its own is not the solution to the problems of communication over unreliable multipath channels. The channel time dispersion will excessively attenuate some subcarriers such that the throughput on these subchannels would be unacceptable small. Only if the joint signal of many subcarriers is processed appropriately, the diversity advantages of MCM can be exploited. The need for coding across subcarriers was addressed by Sari et al. warning against overly enthusiastic pursuit of MCM. The advantages of frequencydomain implementations of equalizers (using an FFT) should not be mistaken for an "inherent" diversity gain of OFDM, which may not exist.
Coding for wireless 
In an OFDM transmitter, blocks of k incoming bits are encoded into n channel bits. Before transmission, an npoint InverseFFT operation is performed. When the signals at the IFFT output are transmitted sequentially, each of the n channel bits appears at a different (subcarrier) frequency. Such coding across subcarriers is necessary. If one subcarrier experiences deep fading, this leads to erasure of the bit on this subcarrier.
But of course coding across subcarriers is not the only mechanism that can be invoked to combat dispersion or to exploit diversity. Other possibilities are
Figure: OFDM transmitter using an (inverse) Fast Fourier Transform (FFT).

Figure: Experimental OFDM transceiver at IMEC, Leuven. Slides from IMEC tutorials: intro, turbo coding, turbo for OFDM 
Many channel simulation models follow the narrowband model. Wideband channels are often simulated by extending the model assuming multiple timedelayed resolvable paths. This allows the simulation of the channel impulse response, including its stochastic behavior. To determine the performance of an multicarrier, OFDM or MCCDMA system, another approach can be to model a set of fading subchannels. Considering a single subcarrier, the channel may be modelled as a narrowband fading channel, for instance with Rician or Rayleigh amplitude distributions. The collection of multiple subcarriers can be modelled as a set of mutually dependent fading channels. In such model, it is important to address correlation of the fading of various subchannels using the models of delay spread and coherence bandwidth. See a discussion of such model. Also: read about the discretefrequency model for OFDM with Delay spread and Doppler.
Listen to an MP3 audio program about on OFDM, featuring Jeff Anderson (SONY), Geert Awater (Lucent), Helmut Boelsckei (Stanford U.) and JeanPaul Linnartz (Philips). 
SMIL Synchronous Multimedia 
Animated audio tutorial on OFDM and MCCDMA 