The radiometric techniques that give absolute dating estimates are based on radioactive decay of elements such as uranium. Looking at how rock formations are structured, a geologist may be able to say which rock was developed in which layer in a particular order but not be able to determine that actual geologic age of the layers.
Geologists also have radiometric methods for absolute dating based on radioactive decay of certain elements. A relative date is obtained from stratigraphy For instance you can see pottery styles change as you move to shallower layers in a sequence. Then when you dig somewhere else and find the same pottery in the ground you can say that that layer is the same relative age as the same layer of pottery found at the first site.
Short Answer: The term relative dating is distinguished from absolute dating to make it clear that one does not get a specific estimate of the age of an object from relative dating, but one does get such an estimate of true age from absolute dating.
There are several techniques employed in both sets of methods. Long Answer: Sciences such as geology, paleontology and archeology are very interested in identifying the age of objects found and these scientists sometimes use both relative dating or absolute dating to characterize the age of the objects they study.
Wind and water erode strata and some areas are uplifted or even tilted.