Temple of Castor and Pollux
The Temple of Castor and Pollux is at one end of the Roman forum, near the Arch of Septimius Severus. During the battle of Lake Regillus against the Etruscans, in 496 BC, some people said they had seen the twin gods Castor and Pollux helping the Roman side. After the Romans won the battle, they decided to build a temple to honor the gods who had helped them. That temple was finished in 494 BC, in the early Republic, and when it got old it was rebuilt in 117 BC, and restored again in 73 BC.
But the picture here is of a later temple on the same place. The original temple had burned down, and Tiberius built a new one in 6 AD. So the Corinthian-style temple you see today is from the Julio-Claudian period. This temple was used to keep some of the imperial treasury in (the emperor's money).
But most of the marble has been stolen away for other buildings since the fall of Rome, and only these three columns are left now (and even these had to be put back up because they had fallen down in an earthquake - see where they have been mended?).
Learn by doing: build a Roman temple out of Lego
More about the Roman forum
The Colosseum and the Roman Forum, by Martyn Whittock (2002). Easy reading.
The Roman Forum, by Michael Grant (1970). Out of date, but Michael Grant is an entertaining writer with a simple style which teenagers may appreciate.