The Forum was built in the 2nd century BC and is a collection of buildings. Not a lot is left, but it was enough for archaeologists to figure out this was a meeting place for important citizens in Cosa.
In the mess of stone, you can make out walls and doors, as well as a number of support pillars. To the west is a Byzantine basilica, which was built, along with the adjacent cemetery, in the 6th century.
Next to the basilica, separated by a narrow road, are the remains of the curia and comitium.
The comitium was a gathering place for citizens. The curia was a smaller version of the Senate of Rome and was where the men who governed the city gathered. A few ruins of a temple, dedicated to Concordia, sit nearby.
The planning of the Forum is important because it says a lot about the mindset of the Romans. Not an organic collection of buildings, the town planning paid mind to both religious requirements, mostly that everything had to culminate around a temple, and state needs.
Of course, it isn't exactly easy to discern temple from meeting place any more, but the curators have included information panels to describe each building and the history behind it.