Q: Do strings wear because of the tension applied to them or any other factor, or do they stay in new condition as long as you don't use them?

A: There are multiple factors that cause strings to degrade over time. Once they are exposed to the air, they will start to oxidize. In places with a dry climate, this is a very slow process, but it will be more severe in locales with more moisture in the air.  Also, when they are tuned to pitch, the constant tension (about 10kg per string) causes strings to slowly lose elasticity, even the guitar is not played.  Despite this, however, if a guitar isn't played much, the strings will last a reasonably long time.

Playing will definitely cause the strings to degrade much faster. First of all, playing increases the stressful tension on the strings, causing them to lose elasticity sooner.  Second, playing increases the wear and tear on the strings from being pressed against the frets.  And third, the bottom three wound strings collect sweat, oils, and gunk on your hands, which then seeps between the windings, down to the core wire, and dampens the string.

String life can be prolonged by wiping/cleaning the strings and by detuning them for storage. Dunlop 65 Ultraglide String Cleaner and Conditioner can help.  Most importantly, washing your hands with non-moisturizing soap before playing increases string life.