Political campaigns have frequently used the online social network as an important environment to exhibit the candidate ideas, their activities, and their electoral plans if elected. Some users are more politically engaged than others. As an example, we can observe intense political debates, especially during major campaigns on Twitter. In such context, this paper presents a characterization of politically engaged user groups on Twitter during the 2016 US Presidential Campaign. Using a rich dataset with 23 million tweets, 115 thousand user profiles and their contact network collected from January 2016 to November 2016, we identified four politically engaged user groups: advocates for both main candidates, political bots, and regular users. We present a characterization of how Twitter users behave during a political campaign through the language patterns analysis of tweets, which users receive more popularity during the campaign and how tweets from each candidate may have affected their mood variation, as expressed by the messages they share.