As per Google's IPO filings (http://www.sec.gov/Archives/edgar/data/1288776/0001193125041...), there are two classes of Google Common Stock, Class A and Class B.
Only Class A stock can be publicly traded. It's listed as GOOG on Nasdaq.
Class B stock is "insider" stock. It's the stuff that Larry, Sergey, Sequioa, and Google's employees got. It can't be publicly traded.
If you want to sell Class B ("insider") stock, you can convert 1 share of Class B stock into 1 share of Class A stock. Class A stock can then be sold on the public market.
The catch? Class B stock gets 10 votes per share, while Class A stock only gets 1 vote per share.
If someone decided to launch a hostile takeover of Google, they'd have to convince Larry, Sergey, and Google's VCs and employees that Google should get acquired. Most other stockholders (those holding Class A stock that is publicly traded) don't really matter, because their stock gets far, far fewer votes. Even if you bought all the oustanding Class A stock there is, you'd still have a rather tiny share of voting power.