These two are a little confusing at first sight, so let's put them down here.
The sample variance, S^2, is an attempt to estimate the popupation variance, sigma^2.
http://en.wikipedia.org/wiki/Variance#Population_variance_and_sample_variance
The standard error of sample mean, on the other hand, tells us the standard error of the sample distribution. That is to say, as we draw samples from a population to estimate the mean, the sample mean itself would form a distribution; what is the standard deviation of THIS distribution?
http://en.wikipedia.org/wiki/Standard_error_%28statistics%29#Standard_error_of_the_mean
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