i have had this question in my mind for a long time. there are various types of plectrums meaning that some are very flexible and then there are ones which are hard. what are their uses? when do i use a soft bendy pick and when o i use a hard one?
Generally thin picks (.15-.64mm) are used for jazz and some blues. Thicker ones (.65-.84mm) are used for most acoustic as well as blues, rock and most other general playing. Thicker still (.85-1.0mm) are what most metal guitarists use, and anything over 1.0mm is really a bass pick.
Really, it all comes down to personal preference. You can use any pick for any style... but yeah, there is a tendency towards certain levels of flexibility in different styles, as Andrew mentioned.
There are also plectra of different materials beyond plastics. Metal, wood, compressed felt, coconut shell, resins, horn, bone, etc etc... all have different attack properties. They're usually not expensive, so you can play around to find out what works for you and your style.