加密服务如何像Boxcryptor和CloudFogger一样工作？

I am familiar with TrueCrypt and the whole volume in which I drop files and then lock everything up, but how do services like Boxcryptor and CloudFogger work? They seem to act like they have file volumes that wrap the files, but then I can move the files anywhere I want (even when logged out) like onto a hard drive and decrypt them on another machine. They even provide thumbnail view, etc which baffles me.I am familiar with TrueCrypt and the whole volu