A sequence is an ordered set of numbers, such as 2, 4, 6, 8, ... (The three dots at the end means the sequence goes on forever in this same pattern.) See the link to MathsIsFun (it's British) for more of the details. Basically what you need to know now is that sequences can be arithmetic, also called additive, or geometric, also called multiplicative.

An additive or arithmetic sequence is one created by adding. To get a new term, you add a constant to the previous term. {2, 4, 6, 8, ...} is a perfect example. Each new term is created by adding +2 to the previous term. (Remember, you can also add negative numbers, which is to say, arithmetic sequences can also be created by subtraction of a constant.)

A geometric sequence is one created by multiplying. To get a new term, you multiply the previous term by a constant. An example is {2, 4, 8, 16, ...}, where each term is multiplied by +2 to get the next term.

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