From Wikipedia.org

Gold and other precious metals are assets that are both tangible and liquid (i.e. easily traded), unlike real estate which is tangible but not liquid, or company shares and bonds which are liquid but not tangible.

Considering its high density and high value per unit mass, storing and transporting gold is very easy. Gold also does not corrode. Historically, it was also very easy to verify that an offered coin had the density of gold through the use of Archimedes’ principle. Today, however, some metals are denser than gold yet cheaper. While some think gold deserves special treatment based on its cultural value and use as money, others consider gold a commodity, like copper or lead.

Information and picture from Wikipedia.org