To the best of my knowledge, when
looking at a system level block diagram of an OFDM
transceiver, the IFFT is contained on the transmitter
side just before the mixer and the FFT block is
on the receiver side just after the mixer.
Really, there is a DAC and LPF (perhaps other
blocks) after the IFFT in the TX and there is probably
some type of AGC plus filters and an ADC after the
mixer on the RX side.
The names of these
blocks stand for Inverse Fast Fourier Transform
(IFFT) and Fast Fourier Transform (FFT). They are
supposed to represent the signal processing which
is done to convert a time domain signal into a frequency
domain signal and back again (which is what must
be done to encode and decode the digital information
into an analog signal).
I'm not 100% sure
what they do, but I do know that they are part of
a DSP algorithm and to find out more information
you can visit a signal processing or modulation
theory book to get a little more information.
I hope this helps a little bit.