General forms of a Sequence:
Sequence:  list of numbers

Arithmetic:  sequence of numbers such that the difference between the consecutive terms is constant.
                      

Arithmetic Sequence	Common Difference, d
1, 4, 7, 10, 13, 16, ...	d = 3	add 3 to each term to arrive at the next term, or...the difference  a2 - a1 is 3.
15, 10, 5, 0, -5, -10, ...	d = -5	add -5 to each term to arrive at the next term, or...the difference  a2 - a1 is -5.
		add -1/2 to each term to arrive at the next term, or....the difference a2 - a1 is -1/2.

Geometric: sequence of numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the common ratio.