The difference between relative dating and absolute dating is that relative dating is a method of sequencing events in the order in which they happened.
Before radiometric dating (or other methods of absolute dating like counting tree rings) it was difficult to determine the actual age of an object.
Radiometric dating, based on known rates of decay of radioactive isotopes in objects, allows a specific age of an object to be determined to some degree of accuracy.
Relative dating is a scientific process of evaluation used to determine the relative order of past events, but does not determine the absolute age of an object.
The circumstances of the object may allow one to say that one object is older than another without being able to assign a particular age to the objects.
Very often historical evidence is found in layers and older layers are further down that the top layers.
For example: If an archaeologist is studying past civilizations, the archaeologist may be able to say that in a particular location the ruins of one civilization were found to have been built on another and so the layers unearthed in an excavation convey the sequence of historical occupations without revealing the actual dates.
However, carbon dating is an absolute dating technique that can give an estimate of the actual age of an artifact and thus an estimate of the age of other objects in the same layer.
Carbon dating is one example of radiometric dating.
Similarly, relative dating is done by paleontologists who find layers of fossils.