Robert was the oldest son of President Abraham Lincoln. He served as Secretary of War under President Garfield and President Arthur. There is a fascinating anecdote on Wikipedia about how Robert was saved from injury or possibly death on a train platform by Edwin Booth, brother of John Wilkes Booth. When Edwin Booth pulled Lincoln back from nearly falling off the platform onto the track of a moving train, he did not know he was saving the son of the man his brother had very recently assassinated.
Robert Todd was also played hotly by Joseph Gordon-Levitt in the movie Lincoln. The resemblence is a little freaky.