30 Rock, on NBC
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Caution: spoilers.

First of all, the title refers to 30 Rockefeller Plaza. Which is where a late-night sketch comedy show, called "The Girlie Show" (TGS) is made. The head writer of this fictional sketch show is Liz Lemon (played by Tina Fey, who is also the creator and writer of 30 Rock itself). I dunno if Lemon is also supposed to be like a producer or what, but she does seem to be in charge of TGS. There's also a producer named Pete Hornberger, and various writers, including Frank Rossitano, "Toofer," Lutz, etc. The star of TGS is Jenna Maroney (Jane Krakowski), though she's kind of... I dunno, maybe a bit ditzy, sort of. An aging ingenue. Though over the course of the series, she seems to get more clearly nuts and self-obsessed and stuff. And there's also an actor named Josh, who mostly does impressions. But anyway, at the start of the 30 Rock series, a movie actor named Tracy Jordan (Tracy Morgan) is added to the cast of TGS. He's kind of crazy, and improvisational, a wild card, a loose cannon... whatever. And he has a couple of friends/entourage called Grizz and Dot Com, who are smarter and saner than he is. And then there's an NBC executive named Jack Donaghy (Alec Baldwin). He interferes with TGS and generally causes stress for Lemon. And so does Tracy. And everyone, really. But Jack kind of becomes Lemon's mentor, which she maybe resents at first, but over the years they become friends, even though they rarely agree on much. Oh yeah, and there's also an NBC page named Kenneth Parcell (Jack McBrayer), who seems rather naive and is very eager to please everyone. He's absolutely hilarious, and over the course of the series becomes kind of more complex and mysterious (possibly much older than he seems), though in spite of being about the nicest guy ever, he somehow manages to be ridiculously sexist and stuff. He's just impossible to peg. And Liz has an assistant named Cerie, who doesn't actually do much except look hot and be kind of dumb. And occasionally there's a quack doctor named Leo Spaceman (Chris Parnell); his name is pronounced Spa-chemin, though Tracy pronounces it Dr. Space-man. There are various story arcs throughout the series, like Jack's rivalry with another exec named Devon Banks (Will Arnett). Also over the course of the series, Liz has a number of different boyfriends. And Jack has a number of different girlfriends. I'm afraid I never kept very good track of what happened when, for most of the series, but maybe someday I'll try to sort it all out. Anyway... the whole series is crazy and hilarious and redonkulous and brilliant.

Season 4
NBC is purchased by Kabletown. Josh quits TGS, and is eventually replaced by someone named Danny, but that's not important. Neither character was ever really important, TGS is basically about Jenna and Tracy. Also, Jenna starts dating a drag queen named Paul, who's crazy in pretty much exactly the same ways she is. Jack dates both his old high school crush Nancy Donovan (Julianne Moore) and a TV journalist named Avery Jessup (Elizabeth Banks).

Season 5
We meet the head of Kabletown, Hank Hooper, who is a family-oriented, upbeat kind of guy. Like creepily so. There are also a few episodes with his granddaughter, Kaylie (Chloe Moretz), who becomes a rival of Jack's. And Tracy's wife Angie (Sherri Shepherd) gets a reality show called "Queen of Jordan." And Jack marries Avery. They have a baby named Liddy. But later, Avery is taken hostage in Korea.

Season 6
Liz gets a boyfriend named Criss Chros (James Marsden), who is better for her than any of her previous boyfriends. Jack spends much of the season trying to get Avery released, but also develops feelings for her mother Diana (Mary Steenburgen). Kenneth leaves the page program, but later tries to get reinstated. He's replaced by a new page named Hazel Whassername (Kristen Schaal), though she's the craziest person on the show. Meanwhile, Kenneth goes through a couple of other jobs at NBC.

Season 7
I'm sure I'm forgetting tons of stuff and leaving out lots of important characters, from throughout the series. (As I said before, maybe someday I'll sort it out.) For now, I don't want to say anything about the seventh (and final) season, but of course it was great, and the series finale was pretty much a perfect end for the show.