To build on what @lvm said, a parameter is a population value. A STATISTIC (which lvm calls a parameter estimate) is a random variable that depends on a random sample that is drawn from a population. The statistic has a distribution, which reflects that fact that if you choose a different random sample, you will obtain a different estimate. The collection of all estimates, taken over all random samples of a particular size, can be described by the probability distribution of the statistic, which is called the sampling distribution.
The standard error of a statistic is the standard deviation of the samping distribution. For simple statistics, the sampling distribution is known asymptotically (that is, for very large samples). For other statistics, you need to assume that the population is normally distributed if you want an approximate formula for the standard error.
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