Physical description: She's a lavender unicorn about three feet tall at the shoulder, maybe a little over four feet tall at the tip of her horn (since she's considered a pony, despite being a unicorn-pony, and ponies are generally smallish horses, we see her as being on the small side), with a violet-blue mane and tail, with a streak of magenta and indigo, with large violet-blue eyes. On the flanks of her rump, she has a "cutie mark", a kind of coming-of-age mark which ponies receive when they discover their innate talent; hers is a pink, six-pointed star surrounded by five smaller, white six-pointed stars.
Personality: Like her handle says, she's a geek. Very fond of learning, very curious about the world, but would rather read about it than actually track it down, but life has its ways of getting her out of the library and into the field. Often has her nose buried in a book to the point that she's not always big on socializing, but her arrival in Ponyville and her meeting with "the mane cast" has taught her that one can learn as much from your friends as you can from books. Generally very pleasant and a little clueless in a "needs experience" sort of way. Can be a bit snarky in a deadpan manner, usually when someone is genuinely being an idiot. Sometimes can be a know-it-all and rather bossy, but she's never mean-spirited about it. Also a bit of an organizing-things freak: she's known to make to-do lists obsessively. She can get comically snippy or (literally) flaming-mad when provoked (and sometimes her patience can wear thin easily). Oddly enough, despite the fact that magic is prevalent in her world, she's somewhat skeptical when it comes to supernatural things like ghosts and spooks (guess she's the Dana Scully of Equestria).
Short biography: She was born in Canterlot, the mane city in Equestria, to Night Light and Twilight Velvet, a loving pair of unicorns, and had a happy foalhood, growing up with her doting older brother, Shining Armor, and with the Princess Cadence as her foalsitter and close friend. Princess Celestia, the ruler of Equestria was her teacher and mentor at Celestia's School for Gifted Students; under her tutelage, Twilight discovered he special talent lay in magic. While still a filly, Twilight was given a dragon egg about to hatch and, as a test, was told to use her magic to help it to hatch. She succeeded, but her spell went a bit wacky (causing the baby dragon to grow to giant size as well as causing random things to transform into other things): her power was great, but it needed to be harnessed. She continued her studies for several years, till she was grown and had settled in an ivory tower of her own, along with her constant companion, secretary and general gofer, Spike, the baby dragon.
During a bit of research, she found an old prophecy predicting the return of Nightmare Moon, formerly Celestia's twin sister Luna who had been the guardian of the moon until she had turned against her sister. Twilight Sparkle consulted Celestia, trying to warn her mentor, but the princess sent Twilight to Ponyville, to help organize the Summer Sun Celebration; it's during these preparation that she meets her future band of friends (Applejack, the farmpony; Rarity, the dainty but sassy fashionista; Fluttershy, the resident friend to all animals; Rainbow Dash, a tomboy pegasus; and Pinkie Pie, the resident jokester and partypony), and while she dismissed the possibility that their friendship would ever been more than a passing thing ("The fate of Equestria does *not* depend on my making friends"), she was soon eating her words: At the climax of the Summer Sun Celebration, when Celestia was to appear and raise the sun, Nightmare Moon appeared in her stead, intending to plunge Equestria into perpetual night. Accompanied by her Ponyville companions, Twilight Sparkle set forth into the depths of the creepy Everfree Forest, where Nightmare had imprisoned Celestia, facing dangers at every turn (some of them illusions cast by Nightmare Moon), and realizing that her madcap band of friends were clearly more than willing to go the distance with her. Together, they were able to harness the Elements of Harmony and succeed both in freeing Celestia and returning Nightmare Moon to her true, gentler form as Luna.
At Celestia's prompting, Twilight Sparkle stays on in Ponyville, learning more about the magic of friendship, taking care of the town's library, and getting into various crazy mishaps (including getting turned to stone by a cockatrice -- and being freed when Fluttershy stares it down and orders it to free her, as well as having her horn turn floppy when she got snarled up in a patch of magic flowers known as "poison joke", since touching them causes one to suffer random crazy effects).
Physical Abilities: She's excellent at organizing things (though she has a tricky time finding books in her library, even though they're well-organized). She's also very well-read and well-educated, plus she likes to research things, almost obsessively.
Superhuman Abilities: Magic, in the form of telekinetics (moving objects; manipulating things that would require hands, such as opening jars/doors/etc. turning pages on books, etc), animating otherwise inanimate objects, and summoning the elements such as small gusts of wind. Her horn glows with an orchid light when she's casting a spell.
Her spells don't always work, and at times blow up in her face (she is still a student, after all, so her power isn't strong enough to sustain a major spell).