Some metrics are discussed in terms of being "Seasonal" or not, so what does that mean?  Put simply, a seasonal metric has some type of observable and consistent pattern that repeats over several time periods. The best examples of this tend to be "counting" metrics, like number of new listings or the number of closed sales.  In most regions, those numbers go up during the summer and down during the winter, as you can see in the chart below.  For these types of metrics, it's best to do Year-over-Year comparisons (using the same month of the prior year).  

   

Non-Seasonal metrics are usually the non-counting metrics, like Median Sales Price. For these metrics, you can use either a Month-over-Month comparison or a Year-over-Year comparison.