Source: Action Comics #1 (1938)
Type: The real deal
Depending on who you want to believe, the world's most famous secret identity may have been crafted by combining the names two leading men, Clark Gable and Kent Taylor, or it might be a tribute to Superman's pulp ancestors, Doc Clark Savage and the Shadow alias Kent Allard. In the stories themselves, Clark is Martha Kent's maiden name. Regardless, the secret identity has evolved over time. Originally, when it was still being authored by its creators, it was pure concept. Clark was who you were, and Superman was who you wanted to be. The meek character (Clark Kent is at the very least responsible for keeping the old-fashioned phrase "mild-mannered" in the modern lexicon) who couldn't get the girl's attention who would imagine himself trouncing the bullies while the same girl admiringly looked on. By the Silver Age, it was pretty clear Clark Kent was just a disguise used by Superman (or Kal-El, at his most alien moments, making Superman just as assumed an identity) to hide among us, or usually, as a farcical element in secret identity stories, Lois Lane screwball plots, etc. After Crisis, the character was humanized and Superman became the mask and Clark Kent the hero's real identity, and that's where he would best shine.
After all, why would Superman fans care about Clark if he's 1) just an act and 2) so different, even antithetical, to Superman? You'd just be waiting for him to rip his shirt off and do his superhero thing. After Crisis, the two personalities became a bit more homogenized because Superman was the "act". Superman's valor and physicality were integrated into Clark's portrayal and the comics acknowledged he was a beefy guy and a bit of a stud, even a high school football star, and not the invisible schmoe past stories tended to make him. There's only so much you can hide with slouching. It was this Clark that eventually got the girl, only revealing his Super-secret to her when it got serious. It was this Clark who was every bit her match in the writing department, not just because he could "cheat" and report on his own exploits or check things out with super-senses, but because he was a good writer. He won a Pulitzer prise and wrote two best-selling novels! It was this Clark who got not just one, but two television shows - Lois & Clark and the Supermanless Smallville - and was even made into an action figure!
Reaching #500 Bonus 75 Years of Superman Video
This is frankly awesome.