Anyone use Zotero? How does it work?

I want to use it to store journal articles, and I need to be able to share them with people using EndNote. I downloaded the Firefox plugin and it is working fine, but I’m a little confused as to how it works…

If I add stuff to my library in firefox on one computer, how will I be able to access it on another computer? I think you are supposed to create an account and use the Sync feature, but there seems to be a difference between data sync and file sync, and I’m not sure what it is. Also, what’s to stop other people on the computer from accessing my info? There doesn’t seem to be a log in/out feature. If I want to access my info from anywhere, I use the file sync feature? Are there free alternatives to this because Zotero has a data cap. Can I set up file permissions to give specific people access to certain files if I want to share things in my library?