Since May of 2015 the public has been made aware of Hillary Clinton's use of a private email server during her time as Secretary of State, and how she used this server for all email correspondence.
An email server is basically a small computer with only one function: your email account. For example, Google has millions of email servers. There are servers for our school (the colorado.edu address), and many other schools and companies, as well. These servers have all your sent, received, spam, saved, and deleted emails. They are located all over the world! Don't worry, your information is (mostly) secure. Google encrypts the information, meaning they basically scramble it into a language only the sending computer and receiving computer can understand.
Google also has a lot of technology to prevent hackers from entering their data stream. But, then again, Google is a multi-billion dollar company that has access to hundreds of thousands of people and many hundreds or thousands of facilities to upkeep their servers.
Clinton wanted a "hillary@clinton.com" email, or something similar, as opposed to "hillary@state.gov" address. She wanted to not have to separate her emails coming to her from multiple accounts. She wanted all her campaign emails, personal emails, and professional emails to come from one source, all of which could be accessed from her smartphone.
This is not illegal. The only stipulation to using a private email instead of the government server is that you have to keep a record of all emails. This allows the government to respond to citizens on Freedom of Information Act requests, since the government is keeping records of the emails.
Clinton did none of this. Her server was unencrypted for a time, didn't have any firewall or hacking countermeasures for a time, didn't notify State of the server's existence for a time, and didn't keep record of all her emails.
For a normal person, this is no serious offense. But, when citizens want to see this information, Hillary effectively circumvented the Freedom of Information Act. She did it so effectively that the Department of Justice had to subpoena some of the emails, because many companies (Associated Press, Vice News, Judicial Watch, Cause of Action Institute, etc.) were taking the State Department to court!
Since her email server was private, experts conclude it was unlikely any highly sensitive information was stolen. Although the logs show access from Serbia, Romania, Germany, China, and South Korea, the same experts conclude they didn't escape with anything important.
Currently, the debate is ongoing as whether or not Clinton knew the information she was sending was confidential before sending. If she did, it could mean jail time (although very unlikely she would actually be jailed). If she didn't, then a slap on the wrist for all future heads of departments to use the infrastructure we have to keep our information safe.
In summary, Clinton wanted to streamline her email process. Unfortunately, she didn't use the correct security measures in doing so, and it's possible information escaped that experts couldn't detect (as hacking is unusually difficult to see, the more complex the software is). This isn't the biggest problem, many people would do the same in her shoes. Everyone wants their job to be easier, not harder, and wants all information at their fingertips.
The biggest issue of this, that the media refuses to cover, is her innate knowledge of Freedom of Information Act requests. Because it was a private server, she didn't have to give any information to the people if she deemed all her emails to be "personal," (which she did). It wasn't until there were enough complaints that the Department of Justice got moving on emailgate.
This occurrence isn't exactly a positive move in the direction of governmental transparency, especially as Clinton moves to clinch the Democratic Presidential Nomination. However, Nelson Mandela was imprisoned for 27 years before making his rise to the presidency for South Africa. Perhaps Clinton is looking at the same campaign strategy if found guilty.
