What are the different types of guitar string keys and how do they affect the sound and playability of the instrument?

The main types of guitar string keys are nickel-plated steel, stainless steel, and bronze. Nickel-plated steel strings are bright and clear, stainless steel strings are durable and have a balanced tone, and bronze strings have a warm and rich sound. The choice of string key can affect the sound, feel, and playability of the guitar, influencing factors such as tone, sustain, and ease of bending notes.