Short answers, pulled from the story.
Sam Weller is a fictional Cockney bootblack and personal servant to Mr. Pickwick in Charles Dickens's first novel, The Pickwick Papers (1836-37). He first appeared in the fourth serialised episode and became the most popular character in the book.
Before Sam Weller appeared, monthly sales of The Pickwick Papers were around 1,000 copies. After his introduction, sales rose to 40,000 copies a month by late autumn of 1837. The Paris Review called this turnaround arguably the most historic bump in English publishing.
A Wellerism is a type of comic proverb in which a familiar sentiment is attributed to an anonymous person caught in an absurd or dark situation, twisting the meaning sideways. The term comes from Sam Weller's distinctive speech pattern in The Pickwick Papers.
Sam Weller and his father Tony speak a form of Cockney English from London's East End as it was in 1836, swapping "v" and "w" in both directions, so that "very" becomes "wery" and "away" becomes "avay." Dickens rendered the dialect so precisely that it was already considered outdated just forty years after the novel's publication.
Sam serves as Mr. Pickwick's personal servant and companion, with their bond compared in the novel to that of Don Quixote and Sancho Panza. Sam's loyalty runs so deep that he has himself arrested to follow Pickwick into the Fleet Prison for debt, and he delays marrying his sweetheart Mary out of devotion to his employer.
W. J. Hammond was among the first to play Sam Weller on stage in the 1837 burletta Samuel Weller, or, The Pickwickians. Later portrayals include Sam Kydd in a 1952 BBC serial and again in 1955, Teddy Green in the 1963 West End musical, Roy Castle in both the 1965 Broadway and 1969 television versions, and Phil Daniels in a 1985 television adaptation.