The chemical symbol for gold is Au.  It is one of the noble metals, meaning it is very resistant to oxidation (rusting).  In its purest form it is yellow in color.  Other colors are made by adding other metals.

Pure gold is quite soft, and is therefore easily scratched.  Gold is mixed with other metals to form alloys.  Gold alloys are harder than pure gold.  The amount of pure gold contained in an alloy is measured in karats.  Pure gold has a grade of 24 karats.  10 karat gold is 10/24 (41.7%) pure gold.   14 karat gold is 14/24 (58.3%) pure gold, and so forth.  Pure gold is 24/24 (100%) pure gold. Here is a table of the common gold karat grades.

White gold is an alloy of gold with other metals to give it a silvery color.  Rose colored gold is also made by alloying gold with other metals, primarily copper.

Gold and platinum characteristics to consider when ordering
10k gold:		Least expensive, but tarnishes.
14k gold:		Less prone to tarnishing than 10k gold.
18k gold:		Very resistant to tarnishing.
22k gold:		Will not tarnish, but not as shiny as 10k, 14k, and 18k.
Platinum:		Doesn't tarnish, but will lose its shine over time.
Rose gold:		Beautiful reddish color.  Available in 10k, 14k, and 18k.
White gold:		Good alternative to platinum. Looks similar, but easier to care for, and less expensive.   White gold is available in 10k, 14k, and 18k, and is the same price as yellow gold.