Justice League begins with the formation of the Justice League, a team of superheroes dedicated to protecting Earth from threats too large for just one to handle. The founding members include the three most well-known characters—Batman, Superman, and Wonder Woman, here depicted as a newbie outsider from Themyscira—characters you may have heard of—Green Lantern and Flash—and a couple you may have never heard of—J'onn J'onnz (a.k.a. Martian Manhunter) and Hawkgirl. Each character brings something to the team, both power-wise and character-wise. Batman doesn't really play well with others, but he comes through when needed. Superman gets knocked out a lot because he's too powerful, but he represents the ideals of truth, justice, and the American way. Wonder Woman can deflect anything you shoot at her with her magical bracelets, but she still misses her mother. Green Lantern—the African-American John Stewart, adding some diversity to the team—is a rather serious, somewhat uptight ex-Marine, but he has a strong sense of duty. Flash—Wally West, not that the Flashes are particularly distinct—provides most of the comic relief, but he also functions as the team's conscience and heart. J'onn Jonnz is a somber alien of great power, but unlike Superman, he does not have a real fondness for humans. Hawkgirl is a sassy, militant Thanagarian, but she is made of so much awesome I crushed on her even though she had giant feathery wings. Each episode—and nearly all episodes are two-parters—features members of the League teaming up to take down a foe. Or foes! They fight everyone from aliens to Nazis.
The writers have decades of comic book storylines to pull from, and the seven superheroes bring with them seven Rogues Galleries and seven backgrounds, so there's a wealth of storytelling potential here. The fun comes in developing their own take on everything and developing their own continuity. While each two-party story is largely stand-alone, like with Superman: The Animated Series, there is a strong sense of a continuing story and continual worldbuilding, as recurring villains and allies are treated as part of the universe and not just characters who show up in some episodes and don't do anything in between. Justice League, though, has a stronger focus on character development as well, allowing the characters' relationships to grow and develop over the course of the series.
After two seasons and 52 episodes, the show becomes Justice League Unlimited, in which the Justice League expands to include dozens of new heroes, offering many more opportunities for team-ups and cameos, especially because the series also switches to stand-alone half-hour episodes. But JLU has more continuity than any other series in the DCAU, as it has the whole DCAU to play with now. The writers don't need to rely as much on the comics since they've built more than enough continuity themselves to tell awesome, exciting stories. For instance, JLU brings to the fore a storyline whose overt seeds are dropped in Justice League but has really been in play since S:TAS, and that's the fact that the government is afraid of Superman and his superkind, and they want to be able to defend themselves in case their sworn protectors ever turn on them. I'm really glad duchessdogberry made me watch S:TAS, as it informs a lot of JL and JLU. There are also a few Batman Beyond crossovers, and some B:TAS villains make appearances as well, but by far, the most-referenced source material is S:TAS, especially because that series seemed to specifically dedicate episodes to League members like Green Lantern, Flash, and Aquaman (Aquadouche is thankfully not in the DCAU League, since he's a jerk). I would consider it essential viewing in order to truly appreciate this series.
It's hard to sum up everything that is awesome about this series in a post, especially because I don't want to spoil anything. I didn't know anything about the plots, and I'm not overly familiar with the DCU. I just knew that it was a quality animated series, and it is. It's smartly written, and it's not afraid to go to dark places and actually kill characters. It examines what makes a superhero, both the good and the bad. These are the people who are willing to sacrifice their lives at a moment's notice if it means that others will live, but these are also the people who operate outside the law and make the kinds of decisions no one should have to make. It doesn't confine itself to one type of story, and ridiculous plots make for far better episodes than you expect. The voice acting is fantastic, and JLU features several Whedonverse actors. Shit tends to get pretty epic on a regular basis, and it induces MUCH CAPSLOCK. One of the recurring villains is a hyperintelligent gorilla. What, I have to throw that in there.
Justice League and Justice League Unlimited are great examples of cartoon superhero adaptations done right. They're more exciting, compelling, and clever than any Hollywood movie is likely to be. It's a great way to become familiar with the DCU, as I'm now much more interested in so many more characters and stories from the source material, although I fear that their comic book histories are so muddled that I'll prefer the DCAU versions.
I already kind of miss them.