Mean, Mode, Median, and Range.

1- Mean:
   It's other fancy name is "average", and it's simply the summation of all numbers in the set divided by the count of the set members.

2- Mode:
   The mode is the number that is repeated more often than any other number in the set.

3- Median:
   The median is the middle value after sorting the set, and it has two ways to calculate one if the data set members count is odd and other for the even count member data set.
    I] For odd count set: the number in the middle after sorting.
    II] For even count set: the summation of the two numbers in the middle divided by 2.

4- Range:
   Simply is the result of subtraction of the largest member of the set and smallest number.

EX:
   Calculate the Mean, Mode, Median, and Range for the following sets:
a) { 1, 2, 2, 4, 5 }
Mean = ( 1+2+2+4+5) / 5 = 2.8
Mode = 2 <Number 2 has been repeated two times>
Median = 2
Range= 5-1 = 4

b) { 1, 2, 2, 3, 4, 5 }
Mean = ( 1+2+2+3+4+5) / 6 = 2.833
Mode = 2 <Number 2 has been repeated two times>
Median = (2+3) / 2 = 2.5
Range= 5-1 = 4