Basilica Aemilia, Rome
When the Romans had any activity they wanted to do in groups, but inside, out of the weather, they generally met in a basilica (ba-SILL-uh-ka).
The inside of a basilica was basically a lot like a modern Christian church or a medieval cathedral: a big hall with columns down the sides to make aisles, as you can see in this picture of the Basilica Aemilia. Sometimes they had a raised platform at one end for the important people to sit on. The floor of the Basilica Aemilia was built of many different kinds of marble, that came from Numidia and Egypt in Africa, from Greece, and so forth, to show all the different places that the Roman Empire ruled.
Basilica Julia, Rome
This is the ruins of another basilica, the Basilica Julia, on the other side of the Roman Forum. (The temple of Castor and Pollux is in the foreground, and the Column of Phocas is on the left). The Basilica Julia was built in the time of the Emperor Augustus, at the beginning of the Roman Empire, and named after Julius Caesar. Can you see the front steps, the side aisles, and the middle wide nave?
One difference from medieval or modern churches is that people usually went into a basilica through a door in the middle of the long side, instead of on the short side.
Inside the basilica, judges heard court cases, or politicians made speeches, or sometimes teachers held classes. Outside, on the steps of the basilica, people sold food or changed money in little booths. When the Basilica Aemilia burned down, some of the money-changers spilled their bronze coins and the fire melted the coins into the marble floor. You can still see them there today.
Learn by doing: build a basilica in Lego or in Minecraft
More about Roman basilicas
City : A Story of Roman Planning and Construction, by David Macaulay (1983). For kids - brilliant! The basilica is an early one.
Roman Architecture, by Frank Sear (1983). The standard college textbook.
The Architecture of the Roman Empire: An Introductory Study, by William MacDonald (1982). Actually not so introductory, but it's got great illustrations that really make the building techniques clear.