Different sizes and shapes of picks can create specific sounds on the guitar. Here are some of the most common.
Thin Picks - These are commonly used for playing rhythm guitar. Because they are flexible, they bend as you strum and help create a flowing rhythm. There also is a percussive sound that is generated that adds a great rhythm effect.
Medium Picks - Medium picks are the most common picks used. You can play rhythm or lead effectively. I suggest new players start with medium gauge picks while getting the feel for the guitar.
Heavy Picks - Heavy picks are widely used for playing lead guitar. Because they have a direct response to your hand movement many players choose heavy gauge picks to shred lead guitar.
Different Shapes - The shape of a pick is very personal. Everyone seems to have a shape that feels best for them. There are small and large picks available. There are even triangle-shaped picks.
Tip Shape - The tip of the pick can be very important. Many lead guitarists use a sharp-pointed pick to help generate more speed while rhythm guitarists tend to want rounded-tip picks to help make a warm rhythm sound.
Pick Material - The two common choices that you have for picks are water based and oil based. Water-based picks absorb water and tend to be easier to grip while oil-based picks are shiny, not porous, and can slip in your fingers as you play.
It is suggested that you have an assortment of different picks to use at different times while you are playing; some performers have been known to have about 100 different picks to choose from when they play.