To my understanding they were a people who lived in what is today Yemen. I've heard that religion also had certain holy cites that would become important to Islam. Their location would suggest that they enjoyed the wealth of trade. Other than this though I know nothing of them.
Who were they? Were they Arabs or a similar people? Their location is by some important sea routes, so were they traders? What was their culture and religion like? And how did a Roman Invasion of the region fail?
I'm sorry I can't give you a more detailed response but my backpack was stolen in Jerusalem and had my book. "Bronze Age to the Coming Age of Islam" by Robert Hoyland should give you lots on information. I don't want to write anything without using it as a reference. I'm quite certain you can download an ebook copy for free from gutenburgproject.org. It's a really great read and you'll learn all about their economic and political importance as well as cultural aspects.
NB: Also written as Sabaeans and suspected to be the biblical kingdom of sheba. Does anyone have anything more concrete?