Short answers, pulled from the story.
Dogberry is the chief of Messina's citizen-police in William Shakespeare's play Much Ado About Nothing. He leads a group of comically incompetent watchmen and is famous for his malapropisms, sometimes called "dogberryisms." Despite his absurd ineptitude, he and his constables accidentally uncover the central villain's plot.
Dogberry was created for William Kempe, a leading comic performer in Shakespeare's theatre company the Lord Chamberlain's Men. The names "Kemp" and "Kem" appear accidentally in place of "Dogberry" in the published text of the play, suggesting the character was so closely tied to Kempe that the names blurred during production.
A dogberryism is a malapropism, a word used incorrectly in place of a similar-sounding word, named after the Shakespeare character Dogberry. Dogberry's pretentiousness leads him to reach for sophisticated terminology that he consistently mispronounces or misuses, producing comic results.
Michael Keaton played Dogberry in Kenneth Branagh's 1993 film adaptation of Much Ado About Nothing. Nathan Fillion later played the role in Joss Whedon's 2012 film version.
In Elizabethan English, the dogberry was the common name for the fruit of the dogwood shrub, Cornus sanguinea, a berry considered lowly and inferior to other edible varieties. Shakespeare chose the name to signal the character's humble station before he speaks a word.
Dogberry's constables overhear a conversation involving Boraccio and Conrade, two men connected to Don John's scheme to ruin Hero's reputation. They arrest the men for the wrong reasons, misunderstanding what they heard, but the arrest eventually leads to a confession that reveals Don John's plot and restores the play's social and emotional order.