IC: Star Wars West Marches game
My take on it is that it's a particular style of sandbox. Obviously the whole aspect of scheduling doesn't come into it when you're playing with an online group, but the idea of players forming a small group and then saying, "We're going to go out and explore the third moon" or "We'd like to chase down the bandits that have been plaguing the outlying farms" still applies. Generally, West Marches-style games are more focused on purposeful exploration rather than just roaming around aimlessly looking for trouble. :) But like a sandbox, there's no overarching plot, just a shared persistent world.
For this game, I plan on giving players a base, either a colony ship or a town, that is considered "safe." Characters can stay in town, but nothing interesting will happen there-- it's just a place to meet other PCs, buy/sell gear, and hang out. I'll probably provide adventure seeds in the form of tasks that the PCs can complete-- explore the third moon, take care of the bandits, etc-- but they're optional. If one of the PCs is Force-sensitive and has a feeling telling her to explore the nearby swamp, that's ok. If a big-game hunter PC wants to go out and look for rancors, that's okay too. There's a lot of player input to the world-- if someone says there's mountains with an ancient Jedi Temple, then they exist. If someone says there's rancors in the swamp, then people better be careful when they're out there. After the task is completed, PCs return to base. They can go out again with the same group, find a new group, decide to open a business, just hang out in the cantina for a while, whatever.
Does that answer your question? It's pretty late for me, so I may not be terribly eloquent. :)