The altar (Latin ara/arae pl.) was perhaps the most important feature of a sacred space or temple (so important, in fact, that I typed the word ‘important’ twice and didn’t even notice ;]). It was here that offerings to the gods were given, with the exception of offerings to chthonic deities.
The ara stood outside of either a temple building, usually in front of the steps, or in some other place such as crossroads (dedicated to the Lares Compitales, or Lares of the Crossroads) or a forest. It was typically dedicated by a wealthy patron who wished to perhaps fulfill a vow made to the deity for whatever reason.
Next to the main altar were often erected smaller, temporary altars to “guest deities” associated with the deity who “owned” the temple.
Altars could be small or large, and they were usually shaped as either a tall block or a cylinder. They were often decorated with motifs such as vines and ox skulls, and bore an inscription with the name of the man who dedicated the altar and the god to whom it was dedicated.
Two of the most famous altars in Roman history were the Ara Maxima and the Ara Pacis Augustae.
The Ara Maxima, or Great Altar, was an altar dedicated to Hercules. It was said that Hercules came to Italy during his travels and rid the area around Rome of a monster named Cacus, who had been stealing and mutilating cattle. Relieved, the people set up an altar in His honor and put the Potitii and Pinarii families in charge of His cult and sacrifices. The altar was located in the Forum Boarium.
The Ara Pacis Augusta, or Altar of Augustan Peace, was dedicated on 30 January, 9 BCE in the Campus Martius. It was built in honor of Pax, personified Peace, and the “Augustan Peace” brought about by Augustus’ army, though it was really a huge piece of propaganda. Just like Virgil’s Aeneid. The Ara Pacis bears beautiful carvings of mythology, deities, and religious processions on its four walls. It was actually commissioned by Mussolini to be rebuilt in the late 1930s. You should look up pictures of the panels if you’re interested: they are fascinating!