The use of fossils in stratigraphy is referred to as
biostratigraphy. It is more accurate than absolute dating which has
large margins of error. Biostratigraphers are still refining how the
fossils can be used, but its use in relative dating is powerful.
It has been noted that species have a life span. If the species is
short lived and abundant over a large area, it is called an index
fossil. So if you find these fossils you can correlate time events.
Marine organisms are the best because they can be found in large
geographic environments and are deposited in many rock units.
Below are some examples of index fossils. Their time range is
graphically displayed in a general manner. Biostratigraphers use much
more detailed information.
here for Index fossils puzzle