Horatian satire (or Horatian tone), following the tradition of the Roman poet and satirist Horace, is characterized by playful mockery, a conversational and often gently instructive tone, and easily accessible language. Man is depicted as the Fool (as opposed to the Knave) - the Horatian tone assumes that people aren't inherently evil, just incredibly stupid. Generally, the satirist includes himself or herself with the object of the satire, instead of assuming the persona of the unblemished critic. Satire which is Horatian in tone is often, but not always, optimistic about humankind's ability to rectify its shortcomings.
All satire, by definition, is a mixture of criticism and comedy or wit. Superficially, Horatian satire places more emphasis on the comedic aspects, but this does not mean that it can not incorporate biting social commentary; quite the opposite, in fact. However, the Horatian critical technique employs subtlety instead of resorting to a direct attack, prompting some to accuse the Horatian tone of an excess of tolerance.
In direct contrast to Horatian tone is the harsher, more antagonistic Juvenalian tone.
A modern (or more accurately, post-modern) example of Horatian tone is evident in the writing of Kurt Vonnegut, Jr., who uses humor as a device to poke fun at human folly, and routinely satirizes himself.
This wu is paraphrased from my memories (as my lecture notes are long gone) of English 437, Satire, taught by Dr. Michael J. Marcuse at the University of Maryland, College Park.