The mean, also referred to as average, of a set of data is the sum of all the data elements divided by the total number of elements. As a mathematical equation it is represented as, , where y represents the data elements and n represents the total number of data elements. In descriptive statistics there are two types of mean used: the population mean, , and the sample mean, .
Mean is often used as a measure of center on a sample of data. Care must be taken when using the mean, because if the data has any extreme values (for example, any skewing on a histogram) the mean will be pulled towards those values. In those cases it would be wise to consult a different device for the measure of center (several can be found in the above link).
The Wikipedia provides clear information on the mean, as well as its differences with the median and several generalizations of the mean which can be useful depending on the data being analyzed. This site provides descriptions and examples of the differences between mean, median, and mode.