An outlier causes the mean to have a higher or lower value biased in favor of the direction of the outlier. Outliers don't fit the general trend of the data and are sometimes left out of the calculation of the mean to more accurately represent the value.
An outlier ranges far from the mid-point of the data. It can be either much higher or much lower than the mid-point. Outliers that are much higher than the mid-point of the data inflate the mean, while outliers that are much lower reduce the mean. The inclusion of an outlier in the calculation of the mean depends upon the reasonable expectation of finding an outlier in the data.
The mean is one type of average. It is found by adding a group of numbers and dividing the total by the amount of numbers present in the data set. Median and mode are two other types of averages. The median is found by listing the numbers in numerical order before identifying the middle number in the group. The mode is identified by locating the number that is found most often in the data set. Outliers have much less effect on the median and the mode of a data set.