- Look, up in the sky! It's a bird! It's a plane! It's Superman!
For truth, justice, and the American Way!
Also known as the Man of Steel or the Last Son of Krypton, Superman is a longstanding superhero from the DC continuum originally created by Jerry Siegel and Joe Shuster.
He is easily one of the most powerful superheroes in that continuum, possessing a wide variety of superpowers including super-strength, invulnerability, heat vision, ice breath, and flight among others. Originally, his powers were limited to super-strength, super-senses, and super-control of his muscles, but sometime in the Silver Age of comics, superpowers just got piled onto him, resulting in the flying, super-breathing hero we know today. After the Silver Age, a lot of ridiculous powers were dropped, such as wall-repairing vision, syrup-cleaning breakdancing, and super knitting.
Originally from the planet of Krypton (and originally named Kal-El), Superman was sent to Earth by his father just before that world exploded, hence his title as the Last Son of Krypton. He was discovered on Earth by a Kansas farmer and his wife, who adopted him and named him Clark Kent.
Superman is a sworn upholder of truth and justice for all, though he is based, as it were, in the American city of Metropolis. He has been described by many as the "Boy Scout" superhero due to his solid moral standards and role as protector of all on Earth regardless of nationality, race, or creed. If he is seen breaking his own moral codes, it is either a badfic and he is a character replacement, it is an Injustice-verse fic or it is the Silver Age and everybody, everything, and every plot is kind of dumb.
He has only three weaknesses: magic, the radiation of a red sun, and the radioactive element known as kryptonite. Because it is Superman's only mundane weakness, that naturally means that kryptonite is ridiculously common in the DC universe to give Superman a challenge in-plot. His powers have been reduced many times in canon to give him a challenge—at the height of the Silver Age he was nearly omnipotent. In the Justice League Unlimited TV show, Superman eventually revealed that he virtually never used his full power because he was afraid that he'd inadvertently cause massive property damage and loss of life if he did.
Superman has historically had a few love interests: Lois Lane, Lana Lang, and some mermaid. Don't ask, it was (again) the Silver Age and dumb.
He is regarded by some as a Canon Stu, but by others as a mythological character like Hercules or Paul Bunyan. In some ways, he established the modern image of superheroes in general, and many writers play with the idea of having powers such as his. When he's written well, he's a great symbol of a single individual's power to help. When he's written poorly, he becomes an illogical doofus.