No, it's not part of a fish! Sometimes called picks (or plectrum in the singular). They are a often overlooked aspect of playability and tone.

We all know that plectra come in all shapes, sizes, colours, materials and thicknesses - but you could be excused for asking "why?". Is there that much difference between them all? In a word: Yup. Many renowned guitarists will tell you that a huge part of their sound is largely due to the plectrum they use.

Shape
There are differences in sound due to factors such as: thickness, edge bevel and flexibility. Generally speaking, a sharp tipped thin plectrum will have a bright, quick attack with a fair amount of "plectrum click". On the other end of the spectrum a thick (say 1.5mm) round cornered plectrum will have a very broad, warm tone with a less pronounced attack and less plectrum noise on the string.

Surface
Many companies offer different grips (from raised lettering to perforations) that make it easier to hold on when your hands start to sweat. A rough surface at the edges of your plectrum will give a more percussive sound.

So what are they made of?

Plastic - The most common plectra these days are made of various types of plastic; nylon, delrin and tortex are three of the most common.

Celluloid -  The world's first plastic, was at one time the material for plectra. Only two manufacturers remain, which has led to scarcity and rising prices. The erratic and flammable character of celluloid (I once had a cigarette and a plectrum in my mouth simultaneously, and when they touched...) has also led to its being largely replaced by other more stable plastics.

Tortoiseshell - Used to be quite common but since a ban on their manufacture and distribution in the mid-70's they are virtually impossible to locate. This is certainly better for the tortoises involved - although many older players lament their disappearance. Tortoiseshell plectra were noted for their high stiffness to thickness ratio, their exceptional durability (imagine using the same plectrum for years!) and their rich complex tone. They were also quite easy to reshape by sanding and filing.

Metal - Some players prefer the brash, bright sound that metal plectra offer. They can be made of copper, stainless steel, aluminum or even a coin (both Brian May of Queen and Billy Gibbons of ZZ-Top swear by them)!

Other materials - Stone, Felt, wood, graphite and other more exotic materials have been used for plectrum manufacture, but tend to be comparatively rare.