These are fossils (or groups of fossils) which are used by geologists and paleontologists to help correlate different events or processes in different parts of the world which happened at the same time in the past.
Species which make good index fossils are able to tolerate a wide range of ecological conditions, are geographically widespread, common, easy to find, and evolutionarily short-lived (that is, they speciated and then quickly became extinct). In general, they are also plankton organisms and very small.
From the science dictionary at http://biotech.icmb.utexas.edu/