Well, See. There is a very Basic Forumla of counting anyone's Net Worth.

Net Worth = Assets - Liabilities

The same formula can also be used to count the net worth of Billionaires. Assets include all the Assets of a Billionaire. Here is a List.

01) Stake (Shares) in their own Publically Listed Companies (Amounts 99% of Total Assets & thus Net Worth).
02) Bonds
03) Gold
04) Real Estate
05) Private Equity
06) Art
07) Antiques & Rare Items
08) Web Propwerites or any other Online Assets
09) Any other Assets

The interesting fat here is that, the 99% of Billionaires Net Worth come from one thing only and that is their stake in their own publically listed companies. All the Billionaires have one thing in common and that is they hold a large bulk of shares in their own or family owned publically listed companies and the valuation of these shares is worth of Billions.

Say for Example, Mukesh Ambani, Chairman of Reliance Industries is World's 7th richest person having Net Worth of US $ 20 Billion (2009). This is because his stake value in his own listed company Reliance Industries is this much...!!!