From the Google corporate blog:
The Google logo has always had a simple, friendly, and approachable style. We wanted to retain these qualities by combining the mathematical purity of geometric forms with the childlike simplicity of schoolbook letter printing. Our new logotype is set in a custom, geometric sans-serif typeface and maintains the multi-colored playfulness and rotated ‘e’ of our previous mark—a reminder that we’ll always be a bit unconventional. The final logotype was tested exhaustively at various sizes and weights for maximum legibility in all the new digital contexts. To guide usage in screen and print, we developed standards to cover all aspects of the logotype including spacing, clearance rules, product lockups, and redline specifications for in-product treatments. The Google G is directly derived from the logotype ‘G,’ but uses increased visual weight to stand up at small sizes and contexts where it needs to share space with other elements. Designed on the same grid as our product iconography, the circular shape was optically refined to prevent a visual “overbite” at the point where the circular form meets the crossbar. The color proportions convey the full spectrum of the logotype and are sequenced to aid eye movement around the letterform.
The above-linked wonky explanation goes on forever.