When a guitar string is open (not fretted), it vibrates and produces sound waves. This is the primary purpose of a guitar string, to create musical tones through vibration.
Stretching, on the other hand, is not something that typically happens to an open guitar string. Stretching usually refers to the process of tuning or adjusting the tension of the string. When a guitar string is stretched, its length and tension change, which in turn affects the pitch produced when the string is played.
To summarize, an open guitar string vibrates and produces sound waves, while stretching is a separate action that occurs when adjusting the tension of the string.