I found this at wikipedia:

Fractional ownership simply means the division of any asset into portions or shares. If the “asset” is a property, the title or deed can be legally divided into shares. In certain instances this is done by creating a “mezzanine structure”, i.e creating a company which owns the property then allowing multiple owners or investors to own shares in the company. Those shares can then be purchased and owned by more than one individual. The reasons for a “mezzanine structure” can vary. Two common reasons are to allow transfer of shares without the need to reflect changes on the title or deed to the property, and for tax benefits.