A bit of column A and a bit of column B. The Greeks began the Roman practice of religious syncretism as early as Herodotus (450 BCE) when he visited Egypt and cataloged their gods with Greek equivalents (Thoth became Hermes, etc). The Romans had a similar system of religious tolerance and syncretism which they used as a passive means of Romanization.

Take Britain, for example. When the Romans took control of Britain, a Celtic goddess known as Sulis had a cult and temple at modern day Bath. The Romans equated Sulis with their goddess Minerva and the cult of Sulis Minerva was born. Forgive the informal tone but it was really a case of "Wait, you worship this god? We totally do too! Let's call the same thing for simplicity's sake!" Within a few generations, the Celts, already used to worshiping Sulis are now familiar with the mythos and cult rituals of Minerva as well, and have been Romanized.

It worked both ways though. When the Romans took over Egypt after the Battle of Actium, the cult of Isis (as a single example) became so wildly popular that shrines to Isis have been found in all corners of the Roman Empire. Similarly, many Roman soldiers were worshippers of Mithras, due to his warlike and inconquerable mythos and promise of salvation. Mithraism also had seven stages of initiation, which mimicked the rank structure of the Roman Army, and provided another comforting aspect to the rank and file.

So the short answer is that the Roman gods were gods, but they were mirrored in other cultures, and they used the similarities to help assimilate conquered cultures.

edit: some spelling.

I would think that would depend on when you are referring to. The history of Rome is a long time, and the idea of worship changed a lot. They worshiped spirits of woods, streams and other natural phenomena.

Later periods saw the rise of the traditional pantheon that most people associate with the Romans, most of this was acquired from conquering other nations. Greece is a good example of this, there is evidence to support the idea that some Gods might have developed independently, such as a ruling thunder God. Romans in this case often made the assumption that they were Roman Gods who the Greeks misnamed.

In the later empire they were a Christian nation, but that was mostly in the Eastern Roman Empire.

Source: As The Romans Did: A Sourcebook in Roman Social History, and Oxford Classical Dictionary. As well as being a classics first year at University of Toronto.

although there is a pantheon of Roman gods, is there really a difference between way the Romans imposed their supremacy through religion versus the way Spain did with the Catholic Church and the missions across it's empire in the New World, or the Muslims as they conquered North Africa and the Middle East and erected mosques?

The universality of these religions (Christianity and Islam) seems to be due to their expansion with an empire rather than some mass appeal.

a good read on the topic is chapter 4 of The Spanish Frontier in the New World by David J. Weber entitled Conquistadors of the Spirit.

You may be interested in reading Fustel de Coulanges's The Ancient City (available here in English translation, and here in the French original), in which a lot is said about the way Greek and Roman gods evolved from being the gods of a single family, of a small group of people, of a city, toward larger concepts, and also how gods could sometimes be several at once under the same name. I don't think I can summarize it in a way that does it justice, but it's an interesting read.

I don't know what modern historians think of Fustel de Coulanges's theses, however.

The general idea about gods were that they are related to a region. So no one should wondering about Roman religious thoughts. You will find traces of this idea in the Bible too, when the different tribes named as a whole Israel worshiped the gods of Canaan too to be secure not to miss anything. The consequence of this idea is incorporating other gods or give up your former god. The transformation of this idea into nowadays idea of an universal god startet with nomadic poeple (Israel) and the god was related to a nomadic tribe. But when a god is everywhere, because the tribe was nomadic god became eventually a universal god. Later jewish communities were spreading across the Roman empire because they were popular with this idea. This caused arguing about becomming jewish between jews. Orthodox jews continued with the idea that god is for poeple from Israel only. You will find arguing about this in the Bible too. This book is a nice withness about former ideas.