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Although frequently described as a typographic atrocity committed against a helpless world, Vincent Connare's Comic Sans is one of the fonts most widely used by the general public, who love its artless, jaunty feel. Users love it as much as typographers and designers hate it.
Comic Sans is an (extremely) informal handwritten script which approximates a child's scrawl, and was developed specifically as a visual element for Microsoft's long-forgotten Bob, an attempt at a "friendlier" version of the Microsoft Windows user interface. Connare has since made it clear he never intended the font for general use. From the name it might be inferred that it was modeled after comic book lettering, but comic book lettering is generally more formal / regular, and this was not Connare's intent. Apple Computer honored the popularity of Comic Sans by commissioning the design of Chalkboard, which bears a very strong resemblance to Comic Sans while signally failing to be an improvement.
Whether it is ever appropriate to use Comic Sans is a hotly-debated matter in typographic circles, including Typophile (with the dominant sentiment being, "No. Never. Not ever."), but in practice it is frequently used for party invitations, lost pet posters, inter-office memos and anywhere else a designer (professional or otherwise) wants lettering that is legible but still eye-catching.
Read Vincent Connare's own account of the development of Comic Sans. You might also want to read an interview in which Vincent says "If you love it (Comic Sans), you don't know much about typography and if you hate it you really don't know much about typography either and you should get another hobby."
Starting November 30th 2007, Comic Sans was the featured face on Typophile http://typophile.com/node/39634