One type of relative dating is changes in artifact styles. The style of many artifacts changes through time, even though the function remains the same. We can see this today as styles of cars or clothing change regularly. If you have a photograph of a person, and know when their style of clothing was popular, you can tell when the photograph was taken.
For archaeologists, the changing styles of pottery and projectile points provide the best known sequences. Different styles have been found in different layers of sites, so based on stratigraphy, we can tell the order in which the styles were popular. Once we know where one style belongs in time, any time we find an artifact of that style it dates the site where it is found. Archaeologists call these diagnostic artifacts as they are indicative of a particular time and/or cultural group.