Russians are somewhat formal in direct address and especially if you do not know them very intimately. I would refer to them as Gaspazha Soandso and Gaspadin Soandso unless told otherwise by them.
Mr - Gospodin - pronounced as "gahspahdEEn"
Mrs - Gospoja - pronounced as "gahspahjAh"
Thanks very much. What does that mean literally? It sounds as if it's related to the word for God; is it?
Rather, to the word "Lord." More precisely, "Lord" as applied to Christ is Господь (GospOd', with the stress on the second syllable and with a softened "d"). The "vocativus" derived from this word is Господи (GOspodee, with the stress on the first syllable); for example, Russians say Христос - Господь (Christ is Lord), but Тeбe, Господи (to You, o Lord). The term "gospodin" is very close to the term "Gospod'," but used in a secular context.
BTW, Ms. Hoorah, I would not call parents of my significant other "Gospodin" and "Gospozha." That would sound very cold, way too formal for a Russian ear.
Just call them by their first names, maybe with patronyms. Of course, say "Vy" (a respectful form of "you"), never "ty."