Ponyville's most well known socialite, Pinkie Pie is the embodiment of Laughter, the Element of Harmony she represents. Free spirited and always determined to make her friends smile, not even the laws of physics can get in her way, if she doesn't want them to.
Pinkie Pie was originally designed to be a completely revamped incarnation of a 1st Generation Pegasus named Surprise. However, later in production her wings were given to Fluttershy and Pinkie was made into an Earth Pony. Eventually, she lost her original color scheme and adapted one of a 3rd Generation pony, who she adopted the name "Pinkie Pie" from. Still having a recolored cutie mark and similar personality to Surprise, she remains an amalgam of the two characters.
The Cutie Mark Chronicles
Reciting the story of how she earned her cutie mark, she recalls living on a rock farm in her youth, with her two sisters and parents. Apparently mining and distributing rocks to and from different fields, she admits to have had a very dull childhood, absent of smiles and parties.
While moving the rocks around herself one day, she witnessed a sonic rainboom (later credited to Rainbow Dash). The bachlash of the explosion blew her hair into a fuzzy mess, resembling her default hair style from then forward. She smiled at the bright rainbow more than she has with anything in her life, and she was determined to find a way to share this feeling with everyone.
She organized a surprise party inside her barn, inviting her family to come. Either here she invented or coined the term for "party", since she appeared to have invented it there. After being overwhelmed with the bright colors, music and food, her family finally began enjoying themselves, probably for the first time. Here, her cutie mark appeared, symbolizing the joy she can bring to others, namely through parties. As she ended the story, she proclaims "That's how Equestria was made".
Friendship is Magic: Part 1 & 2
In her first appearance, she's the first pony Twilight Sparkle encounters when visiting Ponyville. When Twilight attempts to start a conversation with her, she gasps dramatically and runs away. Later in the episode, when Twilight makes her way back to the library she would be staying at, she finds that Pinkie Pie threw a surprise party for her to welcome her. Despite Twilight clearly being annoyed, she elaborates how surprised she was when she met another pony in Ponyville she's met that she wasn't friends with yet. So, she threw the party for her to make her feel at home and welcomed, specifically inviting the other four friends Twilight has made since their first encounter.
In the wake of Nightmare Moon's return, she once again meets Twilight back at her library. Pinkie helps Twilight find the book on the "Elements of Harmony", to help fight the ancient evil that's returned to plague Equestria.
Accompanying the others into the Everfree Forest to help Twilight, she saves them from their fear of the darkness and ominous faces marked on the trees. Singing a song that her "Granny Pie" taught her to overcome her own fears, Pinkie uses the power of laughter to make it through the darkness; one of the obstacles Nightmare Moon set in place for them.
Playing little part for most of the episode afterwards, she, along with Twilight's other friends, come to rescue her when she's confronted with Nightmare Moon. Discovering how much their company meant to her, Twilight sparked the elusive sixth element of harmony; the Element of Magic. Becoming one by their friendship and the Elements themselves, Pinkie is then designated the Element of Laughter as with the others. The power from the Elements aligned, they were able to cast out Nightmare Moon's wicked form, back into Princess Luna.
In honor of Luna's return and her being reunited with her elder sister, Princess Celestia; Pinkie Pie quickly got over her distress and threw a party for her the next morning.
Swarm of The Century
Ponyville is plagued by a swarm of asexually reproducing insects called Parasprites, which Pinkie appears to have full knowledge of. During the episode, her friends are constantly trying out or finding solutions to contain and get rid of the infestation, while Pinkie is looking for instruments. Ignoring Pinkie's odd requests, her friends neglect to help.
At the end of the episode, she put her musical abilities to work, and lured the Parasprites out of Ponyville with polka music. Incidentally, she was playing the same song that she played at the first party she ever threw for her family. Given that and her knowledge of the vermin, it's very likely she once encountered an infestation on her old rock farm herself.
Party of One
After seeing how much her friends enjoyed her company at Gummy's birthday party (Gummy being Pinkie's toothless pet alligator), she makes plans to throw a second birthday party. However, her friends appear to be avoiding her invitations and making up plans otherwise, which makes Pinkie very anxious. Becoming paranoid, she interrogates Spike to understand why her friends are avoiding her. Taking her demands out of context, Spike tells her what she wants; that her friends are avoiding her and no longer value her friendship.
Heartbroken and angry at her friends, her hair looses it's natural puffy curls and straightens out again, as it was when she was a kid. Becoming extremely delusional and exhibiting basic signs of paranoid schizophrenia, she throws her party anyways... But replaces her friends with inanimate objects, which she uses as puppets to justify her grief. Rainbow Dash, wandering in looking for Pinkie, interrupts her mad tea party to bring her to Applejack's barn. Her inanimate friends provoking her behavior, Pinkie demands Rainbow Dash to leave, under the impression she's going to get rid of her. Despite her resistance, Rainbow Dash drags her out anyways after failing to reason with her.
At the barn, Pinkie finds that her friends were throwing a party for her. Surprised at first, Pinkie then deduces that it must be a going away party for her, which she objects to. Her friends then explain themselves to her, telling her that it's actually her birthday party. After the conflicting argument against her suspicions sinks in, her hair poofs up back to normal, and she apologizes then thanks her friends for being so considerate, even when she didn't think she could trust them.
Powers and Abilities
Despite being a seemingly average Earth Pony, Pinkie Pie exhibits a number of unnatural abilities that can defy the laws of physics.
"Pinkie Senses" - One of her abilities is a precognitive sense, which she can interpret to tell the immediate future. It acts much like a Spider Sense, however she can specifically interpret what would be happening, whether it's an object falling from the sky, rain, a ditch, etc... Her body makes subtle abnormal patterns that she's figured out how to predict danger from... Even multiple things happening at one. Under scientific evaluation, Twilight Sparkle couldn't diagnose any reason for this occurrence. However, given how reliable it's been, she's learned to trust them anyways.
"Toon Force" - In a broad number of situations, she's been able to mimic multiple cartoonish impossibilities, apparently being the only character to do so. She's been able to miraculously appear out of places that'd be impossible for her to find herself in otherwise, much like Pepe Le Pew when chasing down Penelope Pussycat (even casually hopping like him too). Pinkie can also slow herself down and fall gently from mid-air, walk in mid-air as she would on the ground, stretch her neck cartoonishly long when getting in somebody's face, and in one occurrence she could swim through land.
"Cosmic Awareness" - On several occasions, she's been able to manipulate the closing shot where the screen fades to black, either to have one last line while watching at the audience, to finish another thought or as a foreshadow of the future.