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Eldarerathis summarized it very well. To add some things: The 1.0 version was internally called Petit Four by Google, and that's how it all began. Google is installing a giant pastry on their lawn at Mountain View each time a new version is about to be launched.

You can see pictures of that display at different stages here:

Overall display (Cupcake to Eclair, with Petit Four tribute)

Overall display (Cupcake to Eclair, with Nexus One)


Eclair, Froyo, Gingerbread, Honeycomb and Ice Cream Sandwich