Archaeology only deals with the human past. We call the family of species we belong to hominids and the earliest remains of hominids are about 2 million years old. Archaeology does not deal with dinosaurs or other similar fossils. Dinosaurs became extinct about 65 million years go. From the beginning of the human race millions of years ago to the remains of 20th century industry, archaeology provides us with information such as the beginning of agriculture, the origin of towns, or the discovery of metals and often these can only be understood through the examination of physical evidence.
Archaeologists divide time into periods in order to make it easier to understand.
So archaeologists don't just go round digging holes into the ground. But it can also be destructive, so lets look at other methods of retrieving the information we want. Crops form different patterns if over buried sites, for example crops built over buried walls do not grow as well as those in the surrounding area, while crops growing over pits and ditches will be noticeably taller. The data can be plotted to provide a plan of underground features such as hearths or ditches. There are many books available describing the methods archaeologists use to retrieve the information they want.