WordPress mu is the multi-user version of WordPress. We consider it as a genius invention.
Since it is possible to run a single webshop with WordPress, therefore WordPress mu allows for multiple stores in one installation.

Why would you be interested in this?

• 1. This is esp. interesting for you if you run several webshops, but you dont want to install WordPress several times.
• 2. Instead of running webshops yourself, you rent out webshops to others and earn your money like this.
  This could be a flat rate or a percentage of the turnover of your client stores.

In other words: multiple stores can easily multiply your internet income.