What time is it?
Come on, grab your friends
We'll go to very distant lands
With Jake the Dog and Finn the Human
The fun will never end, it's Adventure Time!"
"Hey, do you know what time it is?"
"No! Time for seven minutes in heaven!"
The creator of Adventure Time is Pendleton Ward, who incidentally has the best name ever. He worked on The Marvelous Misadventures of Flapjack and if you've seen the two shows, you might recognize the similarities in storyboarding and humor. The show has a lot of adult humor that manages to get past the censor, to the point where sexy is a word that is used every few episodes pretty blatantly.
The best way to describe Adventure Time is to say... well, it's weird, but crazy awesome. It's a barely coherent post-modern mishmash set in an adorably sugary, badass post-apocalyptic land of Ooo, a world of nonsensical magic (though no one seems to realize they're living in a post-apocalyptic world).
Characters, clockwise from the bottom:
Jake, a dog who can talk, shapeshift, and stretch his limbs. Best friend and adopted brother of Finn. NOT Jake's pet.
Marceline the Vampire Queen. She's pretty cool and doesn't drink blood, only the color red. She literally drains the color red from things. Probably has a maxed Prankster's Gambit.
Princess Bubblegum, the ruler of the Candy Kingdom and Finn's love interest. In the first episode, she creates a concoction to bring the dead back to life using SCIENCE!
Ice King, the designated recurring antagonist whose crown gives him ice powers. Sometimes he's a jerk but he secretly just wants to be friends. Probably.
Finn, the only living human left in the Land of Ooo. Prone to berserker rages and is fairly competent at beating the crap out of evil things. Sometimes he's kinda dumb. Best friend and adopted brother of Jake.
The Snail: a snail that appears in every single episode, sometimes only for the briefest of seconds.
Tree Trunks, a tiny elephant who acts like an old lady. She is a bit crazy about apple pies. Spoiler:
Shelby, a worm that lives in Jake's viola.
Lumpy Space Princess, a fan favorite character who lives in space, is a princess, is lumpy, and talks like a warped valley girl. Voiced by Pendleton Ward. Just type her name into Youtube and you'll see why she's a fan favorite.