Michelle Obama recently started a new Twitter handle @FLOTUS which saw over 25,000 followers in a few hours' time. This marks her second Twitter account, the other being @MichelleObama which has been used since late 2011, gaining 2.6 million followers in that time.
There is a distinction: @MichelleObama is a political campaign account run by staffers at Obama for America (OFA), Barack Obama's presidential campaign. On the other hand @FLOTUS is an official government account for the first lady of the United States.
The difference is that @MichelleObama tweets are considered messages from Michelle, the presidential candidate's wife, not Michelle as the First Lady of the United States. That's why @MichelleObama tweeted only six times since the end of the campaign, half of them retweets from the @BarackObama campaign account.
@FLOTUS tweets are, by contrast, messages from Michelle as first lady, a very public role that represents the White House in her public appearances and comments, including her tweets. @FLOTUS tweets, as White House communiques, will also be legally required to eventually be archived in a publicly-accessible fashion under the Presidential Records Act.
The different accounts for candidate's wife Michelle and first lady Michelle gives options to choose from whichever platform is most appropriate for her message. President Barack Obama and Vice President Joe Biden are in similar situations – as the President, Obama uses @WhiteHouse though as a candidate, Obama used @BarackObama, while Biden's campaign account is @JoeBiden, his official Office of the Vice President of the United States account is @VP.