Is there a difference between being polygamous and polyamorous?
Yes. "Polygamy" is a word used to describe specific societies where plural marriage is part of the social, religious, economic, or political practice. It is most commonly used in sociological or anthropological terms.
"Polyamory" is a contemporary word for freely chosen, consensual non-monogamy, with no cultural coercion. It is considered inaccurate and even offensive to refer to "polyamory" as "polygamy," since the associations with "polygamy" are often restrictive, cult-like environments where wives are treated as property.
There are some polygamous societies that are not restrictive and cult-like; and technically the words mean basically the same thing - but "polygamy" is not a word that most polyamorous people use because it has other connotations.