The above picture shows some ancient Roman altars, on display in the Scottish National Museum. They are about the size and shape of tombstones, and usually include a circular indentation in the top wherein offerings were left.
Pictures or writing indicating the intended deity are on the lower part of the altar.
In the Roman religion, the altar, more often called by the Latin term ara, a word of uncertain derivation, but which many historians trace to ardeo ("burn"), is generally quadrangular in shape. Vitruvius the architect prescribed that the altar of a temple should be facing east and in a lower position than the statue of worship, so that those who pray look up towards the divinity. The height of the altar, however, varied depending on the type of deity to which it is dedicated: those of the celestial deities, such as Jupiter, were very high, while those of the terrestrial and marine deities and even of Vesta were quite low.
Although in houses the function of the altar was served by the lararium, notwithstanding this you can find cases of real altars.
Picking up on a prescription reported by Macrobius, the modern Roman Way to the Gods states that if one grasps the altar with one's hands, even mere speech constitutes sacrifice, provided a regimen of purity has been observed during the three days preceding the rite.