Why does string strummed with finger sound different from the one strummed with pick?

Simply put, the pick is stiffer than your finger, and it comes to a sharper point than your finger does. If you look at the initial shape at the exact moment of release, the picked string is going to come to a sharper point than a finger-plucked string.

1. When strummed Why do different strings on a guitar have different sounds?
2. Why does my guitar sound bad when I use a pick?
3. Why does my picking sound bad?

When strummed Why do different strings on a guitar have different sounds?

A string that is under more tension will vibrate more rapidly, creating pressure waves that are closer together, and hence have a higher frequency. Thicker or longer strings, on the other hand, vibrate more slowly, creating pressure waves that are farther apart, and thus that have a lower frequency.

Why does my guitar sound bad when I use a pick?

It sounds like you're playing chords by plucking strings one at a time. Unless you're playing arpeggios, that's not the best way to do it: Try strumming them all in a row, in a fluid motion. Keep in mind that some pick noise is inevitable, even desirable.

Why does my picking sound bad?

Ordinarily, a pick is feathered (rather like the windscreen wipers on a car), so it glides more smoothly over the strings. Heavier picks will often make more noise than thinner ones. This presumes strumming is of more than just one string - usually most of them.