Jerry Seinfeld (upper left); Jason Alexander (upper right); Michael Richards (lower right); Julia Louis-Dreyfus (lower left)
- Jerry Seinfeld (Jerry Seinfeld) – Jerry is a "minor celeb" stand-up comedian who is often depicted as "the voice of reason" amidst the general insanity generated by the people in his world. The in-show character is a mild germaphobe and neat freak, as well as an avid Superman, New York Mets and breakfast cereal fan. Jerry's apartment is the center of a world visited by his eccentric friends and a focus of the show.
- Elaine Benes (Julia Louis-Dreyfus) – Elaine is Jerry's ex-girlfriend and later friend. She is attractive and genial, while also being humorous, arrogant and occasionally impulsive. She sometimes has a tendency to be too honest with people (usually by losing her temper), which often gets her into trouble. She usually gets caught up in her boyfriends' quirks, eccentric employers' unusual behaviors and idiosyncrasies, and the maladjustment of total strangers. She tends to make poor choices in men she chooses to date and is often overly reactive. First she works at Pendant Publishing with Mr. Lippman, is later hired as a personal assistant for Mr. Pitt, and later works for the J. Peterman catalogue as a glorified assistant. Elaine is popularly described as an amalgamation of David's and Seinfeld's girlfriends during their early days in New York as struggling comedians.
- Cosmo Kramer (Michael Richards) – Kramer is Jerry's "wacky neighbor". His trademarks include his humorous upright pompadour hairstyle, vintage clothes, and energetic sliding bursts through Jerry's apartment door. Kramer was heavily based on a neighbor of David's during his amateur comedic years in Manhattan. At times, he appears naïve, gullible, and ignorant, and at other times, intelligent, understanding, and well-read; similarly, he is exaggeratedly successful, socially, with his charisma and laid-back personality. This is seen in his success with women and employers. He has been described as a "hipster doofus". Although he never holds a steady job, he is rarely short of money and often invents wacky schemes that often work at first then eventually fail. Kramer is longtime friends with Newman, and they work well together despite their differences.
- George Costanza (Jason Alexander) – George is Jerry's best friend, and has been since high school. He is miserly, dishonest, petty and envious of others' achievements. He is depicted as a loser who is perpetually insecure about his capabilities. He complains and lies easily about his profession, relationships and almost everything else, which usually creates trouble for him later. He often uses the alias Art Vandelay when lying or concocting a cover story. Despite these shortcomings, George has a sense of loyalty to his friends and success in dating women and eventually secures a successful career as Assistant to the Traveling Secretary for the New York Yankees.
Many characters have made multiple appearances, like Jerry's nemesis Newman and his Uncle Leo. In addition to recurring characters, Seinfeld features numerous celebrities who appear as themselves or girlfriends, boyfriends, bosses and other acquaintances. Many actors who made guest appearances became household names later in their careers, or were already well known.
Many Seinfeld episodes are based on the writers' real-life experiences, with the experiences reinterpreted for the characters' storylines. For example, George's storyline, "The Revenge", is based on Larry David's experience at Saturday Night Live. "The Contest" is also based on David's experiences. "The Smelly Car" storyline is based on Peter Mehlman's lawyer friend, who could not get a bad smell out of his car. "The Strike" is based on Dan O'Keefe's dad, who made up his own holiday—Festivus. Other stories take on a variety of turns. "The Chinese Restaurant" consists of George, Jerry and Elaine waiting for a table throughout the entire episode. "The Boyfriend", revolving around Keith Hernandez, extends through 2 episodes. "The Betrayal" is famous for using reverse chronology, and was inspired by a similar plot device in a Harold Pinter play, Betrayal. Some stories were inspired by headlines and rumors, as explained in the DVD features "Notes About Nothing", "Inside Look", and "Audio Commentary". In "The Maestro", Kramer's lawsuit is roughly similar to the McDonald's coffee case. "The Outing" is based primarily on rumors that Larry Charles heard about Jerry Seinfeld's sexuality.
The series was often described as "a show about nothing". However, Seinfeld in 2014 stated "the pitch for the show, the real pitch, when Larry and I went to NBC in 1988, was we want to show how a comedian gets his material. The show about nothing was just a joke in an episode many years later, and Larry and I to this day are surprised that it caught on as a way that people describe the show, because to us it's the opposite of that."
Seinfeld broke several conventions of mainstream television. The show offers no growth or reconciliation to its characters. It eschews sentimentality. An episode is typically driven by humor interspersed with the superficial conflicts of characters with peculiar dispositions. Many episodes revolve around the characters' involvement in the lives of others with typically disastrous results. On the set, the notion that the characters should not develop or improve throughout the series was expressed as the "no hugging, no learning" rule. Also unlike most sitcoms, there are no moments of pathos; the audience is never made to feel sorry for any of the characters. Even Susan's death elicits no genuine emotions from anybody in the show.
The characters are "thirty-something singles with vague identities, no roots, and conscious indifference to morals". Usual conventions, like isolating the characters from the actors playing them and separating the characters' world from that of the actors and audience, were broken. One such example is the story arc where the characters promote a TV sitcom series named Jerry. The show within a show, Jerry, was much like Seinfeld in that it was "about nothing" and Seinfeld played himself. The fictional Jerry was launched in the season four finale, but unlike Seinfeld, it wasn't picked up as a series. Jerry is one of many examples of metafiction in the show. There are no fewer than twenty-two fictional movies featured, like Rochelle, Rochelle. Because of these several elements, Seinfeld became the first TV series since Monty Python's Flying Circus to be widely described as postmodern.
Jerry Seinfeld is an avid Abbott and Costello fan, and has cited the Abbott and Costello Show as an influence on Seinfeld. "Everybody on the show knows I'm a fan. We're always joking about how we do stuff from their show. George and I will often get into a riff that has the rhythm from the old Abbott and Costello shows. And sometimes I'll hit George in the chest the way Abbott would hit Costello." The series includes numerous references to the team. George Costanza's middle name is "Louis," after Costello. "The Old Man" episode features a cantankerous character named "Sid Fields" as a tribute to the landlord on the team's TV show. Kramer's friend is named Mickey Abbott. A copywriter for the J. Peterman catalog is named Eddie Sherman, after the team's longtime agent. In Episode 30, Kramer hears the famous Abbott and Costello line, "His father was a mudder. His mother was a mudder."
Many terms were coined, popularized, or re-popularized in the series' run and have become part of popular culture. Notable catchphrases and terms include:
The lexicon of Seinfeldian code words and recurring phrases that evolved around particular episodes is referred to as Seinlanguage, the title of Jerry Seinfeld's best-selling book on humor.
A signature of Seinfeld is its theme music. Composed by Jonathan Wolff, it consists of distinct solo sampled bass synthesizer riffs (played on a Korg M1 synthesizer) which open the show and connect the scenes, often accompanied by beatboxing. The bass synthesizer music eventually replaced the original piano/synth music by Jep Epstein when it was played again after the first broadcast of the pilot episode. The show lacked a traditional title track and the riffs were played over the first moments of dialogue or action. They vary throughout each episode and are played in an improvised funk style. An additional musical theme with an ensemble, led by a synthesized mid-range brass instrument, ends each episode.
In "The Note", the first episode of season three, the bumper music featured a scatting female jazz singer who sang a phrase that sounded like "easy to beat". Jerry Seinfeld and executive producer Larry David both liked Wolff's additions, and three episodes were produced with this new style music. However, they had neglected to inform NBC and Castle Rock executives of the change, and when the season premiere aired, the executives were surprised and unimpressed, and requested that they return to the original style. The subsequent two episodes were redone, leaving this episode as the only one with additional music elements. In the commentary of "The Note", Louis-Dreyfus facetiously suggests it was removed because the perceived lyric related closely to the low ratings at the time.
In the final three seasons, the bits were tweaked slightly with more frantic rhythms; a bass guitar was added in addition to the sampled bass from earlier seasons. Throughout the show, the main theme could be restyled in different ways depending on the episode. For instance, in "The Betrayal", part of which takes place in India, the theme is heard played on a sitar.