Brass is an alloy. That means two metals are melted and blended together to make a unique new property. In the case of brass, it's any alloy of copper and zinc.
On their own, copper will turn blue, green, or dark and tarnish rapidly. Zinc will also turn very dark. Together, they are much more resistant to tarnish. Brass itself will vary in color from a dark copperish gold to a bright yellow gold. Many people like it's golden appearance for jewelry. Brass is an inexpensive substitute for gold.
Like sterling silver, brass does tarnish, especially in great humidity or air pollution. Just use a soft cotton cloth to polish it to its original shine.
Plating adheres very well to brass, but it will slowly dull and fade away. How fast this occurs depends on the wearer's body chemistry, and the quality and humidity of the air. To help the plating last longer, do not use a jewelry cloth to polish the metal. Just use a soft cloth and wipe clean. Do not use any cleaners either. And the best way to keep the silver plating tarnish free is to store your jewelry piece in a zip lock bag that is airtight.