Unlike the later imperial fora in Rome—which were self-consciously modelled on the ancient Greek plateia (πλατεῖα) public plaza or town square—the Roman Forum developed gradually, organically, and piecemeal over many centuries. This is the case despite attempts, with some success, to impose some order there, by Sulla, Julius Caesar, Augustus and others. By the Imperial period, the large public buildings that crowded around the central square had reduced the open area to a rectangle of about 130 by 50 meters.

Its long dimension was oriented northwest to southeast and extended from the foot of the Capitoline Hill to that of the Velian Hill. The Forum's basilicas during the Imperial period—the Basilica Aemilia on the north and the Basilica Julia on the south—defined its long sides and its final form. The Forum proper included this square, the buildings facing it and, sometimes, an additional area (the Forum Adjectum) extending southeast as far as the Arch of Titus.

Originally, the site of the Forum had been a marshy lake where waters from the surrounding hills drained. This was drained by the Tarquins with the Cloaca Maxima. Because of its location, sediments from both the flooding of the Tiber and the erosion of the surrounding hills have been raising the level of the Forum floor for centuries. Excavated sequences of remains of paving show that sediment eroded from the surrounding hills was already raising the level in early Republican times.

As the ground around buildings rose, residents simply paved over the debris that was too much to remove. Its final travertine paving, still visible, dates from the reign of Augustus. Excavations in the 19th century revealed one layer on top of another. The deepest level excavated was 3.60 meters above sea level. Archaeological finds show human activity at that level with the discovery of carbonized wood.

An important function of the Forum, during both Republican and Imperial times, was to serve as the culminating venue for the celebratory military processions known as Triumphs. Victorious generals entered the city by the western Triumphal Gate (Porta Triumphalis) and circumnavigated the Palatine Hill (counterclockwise) before proceeding from the Velian Hill down the Via Sacra and into the Forum.

From here, they would mount the Capitoline Rise (Clivus Capitolinus) up to the Temple of Jupiter Optimus Maximus on the summit of the Capitol. Lavish public banquets ensued back down on the Forum. (In addition to the Via Sacra, the Forum was accessed by several storied roads and streets, including the Vicus Jugarius, Vicus Tuscus, Argiletum, and Via Nova.)

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From the 17th through the 19th century, the Roman Forum was a site for many artists and architects studying in Rome to sketch. The focus of many of these works produced by visiting Northern artists was on the current state of the Roman Forum, known locally as the Campo Vaccino, or "cow field", from the livestock who grazed on the largely ignored section of the city. Claude Lorrain's 1636 Campo Vaccino shows the extent to which the buildings in the Forum were buried under sediment. Renowned British artist J. M. W. Turner painted Modern Rome – Campo Vaccino in 1839, following his final trip to the city.

The Roman Forum has been a source of inspiration for visual artists for centuries. Especially notable is Giovanni Battista Piranesi, who created a set of 135 etchings, the Vedute di Roma ("Views of Rome"), in which the Forum figured significantly. (Many of the features documented in Piranesi's views have now vanished.)

Other notable artists of the Forum include Canaletto, Maerten van Heemskerck, Pirro Ligorio, Giovanni Paolo Panini, and Hubert Robert.

Other fora existed in other areas of the city; remains of most of them, sometimes substantial, still exist. The most important of these are several large imperial fora forming a complex with the Forum Romanum: the Forum Iulium, Forum Augustum, the Forum Transitorium (also: Forum Nerva), and Trajan's Forum. The planners of the Mussolini era removed most of the medieval and Baroque strata and built the Via dei Fori Imperiali road between the Imperial Fora and the Forum. There are also:
• The Forum Boarium, dedicated to the commerce of cattle, between the Palatine Hill and the river Tiber,
• The Forum Holitorium, dedicated to the commerce of herbs and vegetables, between the Capitoline Hill and the Servian walls,
• The Forum Piscarium, dedicated to the commerce of fish, between the Capitoline hill and the Tiber, in the area of the current Roman Ghetto,
• The Forum Suarium, dedicated to the commerce of pork, near the barracks of the cohortes urbanae in the northern part of the Campus Martius,
• The Forum Vinarium, dedicated to the commerce of wine, in the area now of the "quartiere" Testaccio, between Aventine Hill and the Tiber.
Other markets were known but remain unidentifiable due to a lack of precise information on each site's function.