Starlight is the average size for a pony. Much of her fur is a dullish pink lavender color, with her mane smoothed and tossed to the side as purple with a light green outline within it. Same thing can be said for her smooth tail. Starlight's Cutie Mark consists of what appears to be a star with two streaks of magical light green energy coming from it.
Within the village, her look remained the same with the exception of a fake equality cutie mark on her flank, resembling an equal sign.
In her first appearance, Starlight is presented as a alagory for a cult leader in some aspect: her philosophy being that no one can truly be friends unless they're all "equal", an idiology she had influenced on others within her village. She is shown as a carismatic and level-headed individual, fairly clever and observant of situations around her. Calm, collected, and manipulative, her main goal was to make sure everyone in Equestria sees to her ideals, and follows the path she had created.
Throughout Season 4 through Season 6 since being exposed, her usually calm and friendly nature had ended up being dropped for a more cynical and antagonistic approach to her idiology, which she had grown extremely stubborn over, and refusing to listen to any of Twilight Sparkle's friendship messages, jotting them down as nothing more than her "overblown ego". She had shown to grow easily angered at anyone who tried to tell her otherwise, even attacking them if pushed far enough. It had turn into such a goal just as well to get even with the Mane Six that she even temporarily operated with White Diamond in order to get even with them, and have them see her ideologies, despite being exposed for a fraud.
All this desperation, as revealed in "Butterfly Wings", all stemmed from her childhood friend getting his cutie mark. She was once a friendlier pony, and one that studied extensively on magic, though it was this incident that got her to believe that cutie marks will take one's friends away, and that she never wanted it to happen again, thus sparking her ideology. She seemed to have given up on it at the end of said episode however.
Starlight Glimmer was born in one of the villages in Equestria, gaining a friend named Sunburst. According to her, she and Sunburst had never been apart, her not remembering a time when they were, but that all changed when Sunburst got his cutie mark before she did, and as such, left her. This left her without friends, and she refused to make another one the same way, worried that it'll result in the same conclusion. With this idea in mind, she began to work out her own philosophies that everyone will be fine if they were equal.
And so, she began to spread this message wherever she could, and with this ideology becoming her life goal, she tried her message in multiple places, which included Darastrix, which she apparently was driven out from by the Draconians. After that, she found her village, and used her ideology and a bit of her own magic (something she used on the "stick of sameness") to turn the town into her equality image.
As she became mayor of the town, she was found by Connie II at a point, and taken in so she could manipulate her into her philosophy as well. She was found by the Mane Six, and during their stay tried to show them a similar idea. During which, she presented them the Cutie Mark Vault, where she questioned them on how the subject came up, and exposing them of their true intentions. She then stole their Cutie Marks from them, and had them placed inside one of the village's huts to be manipulated into accepting this ideology. However, she was then confronted by Applejack later on, and ended up exposing herself in a fit of rage to everyone in the village when using her own magic. She tried to convince them still, but when that didn't work, she made a run for it with the Mane Six's cutie marks, until she was stopped by the ponies from her own village. Angered and defeated, she managed to escape through the nearby mountains.
Starlight Glimmer doesn't appear until The Blue Light began to affect Beach City, her then working under White Diamond in order to get even with the mane six for ruining her previous plans. She exposed them to White Diamond, and had them placed in the Human Zoo. She isolated herself in the communication room in order to further mock Twilight Sparkle, revealing she planned to make them all equal similarly to her village so her ideals could be spread, though she was forced to stop when White Diamond found her, and she went to the throne room. When the fight broke out in the throne room however, Starlight retreated and left White Diamond, heading back home.
Starlight only appears once in Season 5. In "Prey for the Beasts", Starlight took control of the Giant Bird, and caused an early winter in Darastrix. This turned out to be her plan to get her revenge on everyone again, and trying to enforce her ideals on Twilight once more after she captured her. However, when the others arrived, she was promptly defeated by Stevonnie, and was forced to leave.
She appears again when she attends Twilight Sparkle's presentation on Cutie Marks at Canterlot, who Twilight thought she had seen amongst the crowd, only to find her in Ponyville. Using an alteration of Starswirl the Bearded's time travel spell, she managed to go back in time to the starting point of Rainbow Dash's Sonic Rainboom, with plans to completely stop it altogether. Her plans were interrupted by Twilight Sparkle and Spike, and Starlight had tried to fight them off, and at a point showed them the source of her reasoning in her backstory. It wasn't until now that she finally took Twilight's message to heart, but upon returning, she found herself in Beach City, something her spell wasn't supposed to do, just to be captured by White Diamond. She was trapped until freed by the others, though ended up caught again by Aquamarine, and brought to the throne room.
She blamed herself for what happened, Twilight reassuring her that this wasn't all her fault, before she and the others did manage to escape. Starlight joined Onion in going to the main power source, and it wasn't until after cutting the power did they get caught by White Diamond again. She managed to get out with the others soon after.
She has developed a very high level of Unicorn magic over her years of study, able to use a various assortment of spells at her disposal. Some of her more notable spells include the ability to fly within a magic aura, and able to alter a pre-existing time travel spell to suit her need.