Grover Cleveland was our only president to serve two non-consecutive terms, making him both the 22nd and 24th president. His terms were broken up by Benjamin Harrison. Interestingly, in Cleveland's elections against Benjamin Harrison, Cleveland won the popular vote but Harrison received more electoral votes, marking one of the few times in history where the chosen president did not match the winner of the popular vote.
Cleveland was the only Democrat in the half-century of Republican political domination from 1861 to 1913, also making him the first Democratic president since the Civil War. At 49 years old, Cleveland became the only President to marry in a White House ceremony , marrying the 21-year-old daughter of a late law associated. Prior to the outing of the relationship, he had pretended this interest was in the woman's widowed mother for the media, making clear that his interest was in the daughter rather than the mother much later on.
In office, he challenged corruption and special favors, vetoing federal funds to Texan farmers. He also vetoed private pension bills as well as pension for disabilities not caused by military service. Furthermore, he investigated government grants thoroughly and made railroads give back 81,000,000 acres that he found misappropriated.
Cleveland saw himself as more of an enforcer, or a watchdog for Congress, than an initiator. Considering he managed to find himself in politics rather than purposely seeking and living for it, he was somewhat overwhelmed by the responsibilities put upon him. For instance, he did not know how to deal with the Panic of 1893, an economic issue, and the subsequent depression. This hesitation led to a rapid decline in support for the incumbent president. Indeed, though the 22nd amendment had not yet been passed to limit an individual to two presidential terms, Cleveland declined to run for a third term.
(Election of 1892, in which Cleveland beat the incumbent Harrison and started his 2nd term)
Sources: Wikipedia, Miller Center, White House.gov, History.com