The Roman Empire came to be from the ashes of the Roman Republic and Imperatorial periods, and once established by Augustus in 27BC, the foundation for the expanded monetary system began and would continue until its overhaul by Diocletian. The base unit of the empire was the copper as, and the system was designed mainly in fractions or multiples of four. The largest regular base-metal coin was the sestertius, valued at four asses, and was made of orichalcum or bronze. The denarius was the silver workhorse and valued at 16 asses. Higher up the chart we find the gold aureus, valued at 25 denarii. In-between and below these denominations we find the quinarius (12-1/2 denarii for gold version and eight asses for the silver module), double-sestertius (eight asses), dupondius (two asses), semis (1/2 as), quadrans (1/4 as) and uncia (1/12 as). In 215 AD, Caracalla introduced a new silver denomination, valued at two denarii. There is a disagreement among scholars as to what it was called, but it will be most often described as an antoninianus or a double denarius.
During the Tetrarchic period, Diocletian overhauled the coinage around 294 and set up a new structure with the base coin having around 5% silver. Today, most call that coin a follis. There were gold (solidus) and silver (argenteus) coins to replace the aureus and denarius, along with fractions of the follis. Constantine the Great reformed the monetary structure again around 318, introducing the siliqua to replace the argenteus, and a base metal coinage system usually referred to now as AE1 through AE4, which describes the size range of the module.
I’ve created a work-in-progress page to give a visual layout of the coinage of the Roman Empire over the different periods: