A plectrum A.K.A Guitar Pick (or a flatpick to distinguish it is from fingerpicks) is a "triangle isocele"

with rounded corners. The use of Plectrums allows for a more focused and agressive sounds in affection with steel strings. many guitarist developped the use of Plectrums remaining with the use of the right hand fingers simultaneously wich is the most dificult but he can use the advantages of both techniques at the same time, this technique is called "Hybrid Picking".
Classical and Flamenco guitarists uses finger nails (sometimes artificial) this techinque allows the player of hitting many string at the same time and in case of the flamenco guitarists it allows them to use the percussive effect, (it originaly works with Nylon strings).
the guitarist shoudn't cut his right hand finger-nails and should take care of them all the time

many guitarists dont use picks even if they are playing rock and specially blues guitarists.
the legend B.B king uses his nails
Eric clapton
Robbie Krieger (The doors)
David Gilmoore (pink floyd) alternates between fingers and Picks
Richie Blackmores (Deep Purple) plays solos with his fingers

Brian May (Queen), he uses a 5p coin. 500 Lira 7agar

Angus Young (AC/DC) occassionaly
Me

even heavy metal songs, though i prefer picks its easier
P.S: advise for beginners, Start with a light Thin Pick that is flexible and buy a whole pack coz you will lose many

.
even those who plays with Picks theres different ones for each style.
Thank you,