What is the difference between the sample mean and the population mean?
Also, population means are parameters while sample means are statistics.
Parameters are fixed, but usually unknown, numbers that describe a population.
In an abstract sense, statistics are really (random) variables. Their values vary as different random samples are taken. Once we take a random sample and compute a statistic, we say that we have "observed a value" of the statistic.
Statistics have sampling distributions which describe, in a probabilistic way, how their observed values vary over many many random samples of the same size from the same population.