A "pick" is a small piece of either plastic, rubber, wood metal or even stone in a triangular shape used to play notes on a guitar. Most rock and metal guitar players use picks. Most of the time they are used on electric guitar, when many people use Finger Picking on an acoustic, or classical guitar.
Picks come in just about every size, and are measured using milimeters. Different people prefer different thicknesses and can be extra thin, to extra heavy. An extra thin pick would be concidered about .40mm or less, when an extra heavy pick would be about 1.5mm - 1.6mm or more. Some picks can even be 2mm or more. Bass guitar picks are much thicker than standard guitar. However, most bassists prefer using their fingers.
Picks can come in hundred of different styles such as skulls, swords, axes, bones, etc. A regular pick is only a triangle with rounded edges. Picks can be dyed in every color imaginable. Yellow, blue, green, pink, turquoise, black, white, and much more. Many picks have an imprint on it whether it is a symbol, company name, or to help grip.
Guitar picks can be a major advantange in playing for you have more of a variety of playing options.
Sweep Picking "Sweeping" is done by holding your pick between your index and thumb, and strum the strings up and down while hitting each individual note without making more than one sound at the same time.
Pinch Harmonics or Artificial Harmonics Pinch Harmonics, or Artificial Harmonics can be played using a pick by holding it with your thumb close to the bottom tip of it, and use a digging or scooping motion with your right hand allowing your thumb to lightly touch the string ringing it in a higher pitch.