Silver tarnishes, which means it reacts to chemicals it touches or substances in the air. Some new silver jewelry is plated with a thin coat of metal to reduce tarnishing and make it appear brighter. When the plating wears off the jewelry will tarnish more easily.
The chemicals to which silver reacts can be any number of things, including oils on the skin.
Tarnish can easily be removed with a silver cleaner or jewelry cleaner.
Inexpensive, gold-plated or "gold tone", jewelry can turn dark gray; and when that happens it's because the plating has worn off, and the inexpensive metal underneath becomes exposed.