Good questions and we could have been clearer about that.  A "composite" is an average of many maps.  So you can take all La Nina winters over the US and then average them together to see what the average pattern is-- this is a composite.  An "anomaly" is a departure from average.  For example, when we make composites, we average together the *anomalies* in snowfall, rainfall, or temperature because we are interested in the question of whether it will be "wetter than average" or "drier than average."  Maps (or composites) of anomalies come in positive and negative values which indicate "above" or "below average."