Definition: is a product that is consumed along with some other product.
Other words
is a good whose use is related to the use of an associated or paired good.
Example of complementary goods
When you go to the mall to get a new laptop you end up buying some software or program to go with it you might be needing a Microsoft office or a special program, the laptop alone won't get the job done you need the complementary software also.
substitute goods
Definition: A product that satisfies the same basic want as another product.
Other words
They are the opposite of complementary goods they are two goods that could be used for the same purpose.
Example of substitute good
Coca Cola and Pepsi they are basically the same thing if one gets more expensive we buy the other.
summary complementary goods and substitute goods are good example to illustrate the difference between changes in demand VS changes quantity demand.