An arithmetic sequence has a common difference, meaning you use addition or subtraction to get from term to term, and each term in the sequence is an equal distance apart.
A geometric sequence has a common ratio, meaning that you use multiplication or division to get from term to term.
If you look at this sequence:
14-2 = 12
12-2 = 10
10-2 = 8
8 -2 = 4
etc.
Because you are subtracting 2 each time, 2 is the common difference, and the sequence is arithmetic.
