The three common measures of central tendency of a distribution are the arithmetic mean, the median and the mode. Think of a distribution in terms of an histogram with many bars; a large sample from a normal distribution would describe a bell shaped curve that is symmetrical. In a perfectly symmetrical, non-skewed distribution the mean, median and mode are equal. As distributions become more skewed the difference between these different measures of central tendency gets larger.
The mode is the most commonly occurring value in a distribution, population or sample.
The mean (arithmetic mean) is the average (sum of observations / number of observations) in a distribution, sample or population. The mean is more sensitive to outliers than the median or mode.
The median is the middle value in a sorted distribution, sample or population. When there is an even number of observations the median is the mean of the two central values.