The hills were really ridges or spurs created by streams running down the edges of the larger valley which had been cut by the Tiber River. The English names for the four main spurs, starting from the north, are the Quirinal (Quirinalis), Esquiline (Esquiliae), Caelian (Caelius), and the Aventine (Aventinus). Each ridge ended in a larger knob or series of knolls two of which had separate hilly identities: the Capitoline (Capitolium) at the end of the Quirinal ridge, and the Palatine (Palatium) connected to the Esquiline by the Velia (cut through by Mussolini to build the Via Dei Fori Imperiali). The Caelian and the Aventine ended in parts simply called major and minor, large and small. Between the Quirinal and the Esquiline was a lesser spur, the Viminal (Viminalis). Various other parts of the Esquiline ridge were also sometimes called 'hills'÷the Cispian (Cispius), the Oppian (Oppius), and the Fagutal (Fagutalis). The Quirinal had several specific heights÷Latiaris, Mucialis or Sanqualis, and Salutaris. Beyond the deep valley on the north side of the Quirinal lies the Pincian ridge (Pincius), which in earlier days went by the name Collis Hortulorum ('Garden heights'). On the right bank, running parallel with the north-south section of the Tiber, is a massive ridge called the Janiculum, from which a small promontory (where S. Pietro in Montorio now stands) juts out toward the river opposite the Capitoline hill. To the northwest of the Janiculum, the hill at the end of a second parallel ridge was called the Vatican (Vaticanus).
Remains of huts and cemeteries dating from the ninth to seventh centuries BC (Iron Age) have been found on the Palatine and Esquiline and in the valley (Velia) between. Other finds suggest that, if excavations were possible on the Capitoline, Quirinal, Aventine, and Caelian hills, early settlement would prove to have occurred there also.
Even the ancient Romans had difficulty deciding which seven hills to count. The Sept_montium was an annual festival on December 11, (normally translated as "seven hills", but perhaps a corruption of soepti montes, meaning "enclosed or fortified hills"). During the festival, each of the "montes" made sacrifice followed by a feast. But eight montes took part: the Palatine, Velia, Fagutal, Subura, Cermalus, Oppian, Caelian' and Cispian, two of which (Subura and Cermalus) are not generally considered to have been hills at all. No mention is made of the Capitoline, Aventine, Quirinal, and Viminal. Later lists suggest that efforts were made to define a new seven, in keeping with the development of the city by the early Empire. A statue set up in the Roman colony of Corinth (in Greece) towards the end of the first century AD showed the goddess Roma seated on seven hills Six are labeled: Palatine, Esquiline, Aventine, Caelian, Quirinal, and Viminal. The seventh was probably the Capitoline (but could also have been the Janiculum). The Regionary Catalogues of the early fourth century AD made yet another selection: Caelian, Aventine, Tarpeian (i.e. Capitoline), Palatine, Esquiline, Vatican, and Janiculum.
For a clear view of what the ridges and hills look like, go to http://www.aec2000.it/aec2000/projects/lanciani/
Click on the squares in the map for more detailed views.
More Rome maps: http://www.geocities.com/Athens/Forum/1274/tiber-and-hills.html
Can't remember the names of the hills? Try this mnemonic: http://www.mmdtkw.org/VHillsMnemonic.html
PS: The Tyrrhenian Sea is the part of the Mediterranean between Italy and Sardinia. Its name is derived from what the Etruscans called themselves.