Does how much someone is worth mean it is how much money they have and also?

According to this link, Sir Alan Sugar is worth £800m. Is that how much money he has? http://www.mirror.co.uk/news/uk-news/rich-list-2012-where-do-the-celebs-chart-811206

Markus Persson sold his computer game Minecraft for 2.5 billion dollars (link below) so is he richer than Sir Allan Sugar?

http://www.belfasttelegraph.co.uk/technology/top-notch-minecraft-creator-markus-persson-sold-his-25bn-stake-to-microsoft-after-one-tweet-31044600.html

Net worth = assets - liabilities

So yes, if a person has a net worth of £800m, that is the difference between his assets and liabilities. It doesn't mean that all of that amount is in cash money though ~ it could be in the form of land, properties (houses), investments, cars, etc.