I GM'd it for about 5 years so I know it fairly well. I'm just failing to describe it as well as I mean to. :)
Basically you had 2 main rule systems for it. Alderac d10, which came first, and which was very similar to the system used in first ed Legend of the Five Rings. Then they came out with a d20 version (Swashbuckling Adventures), which had a campaign settings rulebook to use it alongside the d20 rules. Most or all of the Swashbuckling Adventures sourcebooks have rules in them for the d10 system as well. The d10 system is way better than d20, IMO, and it's what most people online tend to play on the sites I've been on.
The system's not too complicated, and d10 was pretty good except for their mass combat rules, which seemed inordinately complicated to me. A lot of people who ran their own games on other roleplaying sites used to have house rules that adapted the rules in the ways they wanted, and I did that too. I'm curious to try out the new system (I haven't with the advance rules they released yet). It looks like an adapted version of d10 from what I've seen.
Basically the game is set in a fantasy version of 17th century earth called Theah, only with higher technology than that in some places, and it has things like swashbuckling, sorcery, secret societies and politics. You'll find spies, pirates, bandits, and musketeers, and knights, nobles, peasants, scholars, musicians, archaeologists, etc. You have fantasy versions of the larger countries in Europe (France = Montaigne; Spain = Castille; Germany = Eisen; England = Avalon; The Highland Marches = Scotland; Inismore: Ireland etc), and also other countries beyond that like Asia = Cathay, Russia = Ussura, Crescent Empire = The Middle East; and other places. Then you have the world of the sidhe (Bryn Bresail). It blends a fantasy take on the world we know with fairytales, and myths, and stories, and comes up with a game that's all round pretty awesome.