Regarding OFDM (Orthogonal Frequency Division multiplexing)
Preambles are known patterns that allow for easy synchronization.  Preambles are used in signal processing as markers to help synchronize the hardware and software to the incoming data stream which is often too random to find patterns in.  Here's an 802.11 wlan frame.



In real time:


The 10 repetitions of the short preamble give the receiver hardware time to adjust.  During this time signal detection, AGC converges, timing acquisition and frequency acquisition is done.

The two long preambles (with a guard interval before it) are used for channel estimation and finer frequency adjustments before the data can be processed.

The third region of the frame is the Signal field, which consists of one OFDM symbol assigned to all 52 subcarriers. This symbol is BPSK modulated at 6 Mbps and is encoded at a ½ rate. The signal field is used to indicate the rate at which the OFDM symbols of the PSDU payload are transmitted and is not scrambled.

OFDM uses multiple carriers to spread the data out across the frequency band.  This helps reduce problems with signal fading, multi-path and narrow band interference.

Further introductory information can be found here.