A statistic describes a sample, while a parameter describes an entire population. A sample is a smaller subset that is representative of a larger population.
The symbols differ when reporting statistics versus parameters. The average symbol for a statistic is an x with a line on top of it. For a parameter, the average is represented by the symbol μ. Since it is not always possible to collect every single observation in a population, sampling procedures are used to select a representative sample. Random sampling allows a statistician to calculate statistics about a sample and apply them to a population.