One of the crowning achievements of Hollywood, hands down. This is by no means a complete review or summary of the film (for that, I'd recommend the Angry Video Game Nerd, who reviews the film in depth in his youtube films), just a few comments.
For a children's film it achieves the perfect balance of harsh reality and fantasy. New York is portrayed as the grimy crime-toilet that it is. Everyone's poor, and everyone's job and life sucks. Just like real life. Except crazy shit happens sometimes, like having to battle the Foot Clan. Just like real life. (crazy shit happening, not the Foot Clan) April O'Neil tries her best to expose this hidden crime syndicate, but her Establishment shill of a boss is afraid to touch it and eventually fires her for her insistence on exposing the truth.
This film captures the popular culture of the late eighties/early nineties perfectly. The Foot Clan's indoctrination of the youth is a reflection of the gang and urban violence problem which was exploding exponentially at the time. We are shown a world in which evil runs rampant and one of the world's only sources of light and goodness are forced to live underground, lest they be exposed and labeled as 'freaks'. One of the best movies ever.