Department of State employees are supposed to treat any material that even could be classified as extremely sensitive, and not to leave the building. On top of the flagrant disregard for security procedure detailed in this commentary, Clinton routinely shared material with private citizens like Sid Blumenthal, who had no security clearance whatsoever.
For answers to most of your questions about the Clinton email scandal, http://www.thompsontimeline.com/ breaks all the factors in play here down in excruciating detail.
