Short answers, pulled from the story.
A Roman forum was primarily a marketplace where goods were bought and sold, with permanent shops along its edges and open stalls under covered walkways. It also served as a social and political gathering place, hosting debates, elections speeches, and civic assembly.
In new Roman towns, the forum was usually placed at or just off the intersection of the two main streets, the cardo (north-south) and the decumanus (east-west). This central position made it the most accessible and trafficked point in the city.
Every forum included a Temple of Jupiter at its northern end, along with other temples, a basilica, and often Roman baths nearby. A public weights and measures table was also standard, allowing buyers to check that they were not being sold short measures.
The Portico of Pompey, or Porticus Pompeii, was the massive forum built behind the arcades of the Theatre of Pompey, which Pompey Magnus constructed in 55 BC. It became the direct forerunner of Julius Caesar's forum and others that followed in the late Republic.
Roman forum ruins survive across former provinces, including the Forum of Plovdiv in Bulgaria, the Forums of Philippi and Thessaloniki in Greece, the Forum of Beirut in Lebanon, and both colonial and provincial forums at Merida and Tarragona in Spain.
Forum Popili was a forum built at a remote location along a road by the magistrate responsible for that road. When a forum was the only settlement at a site, it gave its name to the entire location, making the marketplace the town itself.