Welcome to PF, Naved.
I'm not sure if there's a distinct line between the two as far as official designations goes. The real engineers probably know.
To me, generally, a screw involves a tapered body that is intended to 'auger' into something like wood or sheet metal. A bolt has a cylindrical body with constant threads that are meant to engage a matching set of female threads in a nut or tapped hole.
Where it gets confusing for me is that a 'cap screw' is actually an Allen-head bolt, and a 'lag-bolt' is a hex-head screw. It almost implies that if you use a screwdriver on it, it's a screw, and if you use a wrench, it's a bolt... but that can't be right.
The biggest difference that I have noticed is that screws come to a sharp point, while bolts tend to have a blunt end. Also, screws tend to be used in situations where there is no hole, or simply a pilot hole, while bolts are used with a complete hole, and use a nut or other threaded reciever on the other end to secure the whole arrangement.
A cap screw does not have a point, nor is it necessarily used with a nut!
I don't think there's a clear-cut difference; I liked Danger's idea of a screw being applied with a screwdriver, but again you'd do up a cap screw with an allen key or socket... And a screw doesn't necessarily cut its own thread either.
Ultimately, I'd say a bolt's threads engage in a nut, where a screw's threads engage within the workpiece itself (either in pre-formed threads, or threads tapped by the screw itself).
Agreed. When posting my answer, I totally forgot about the terms 'set screw' and 'machine screw', both of which would be bolts by the definition that I came up with. Brewskie's is a lot more self-consistent.
In my mind the main differentiation would be size. Anything over about [itex] \frac 3 8 [/itex]" or maybe 1cm is a bolt, smaller is a screw. A taper implies a wood screw or a self tapping sheet metal screw. Machine screws have no taper and come in a wide variety of head styles. Cap screw, button head, flat head, fillister head then you can have within each head style, slotted, phillips, Robertson (Square) and hex heads. To get an education in screws and bolts get your hands on a Machinery Handbook.
From what is published, the term "screw" seems to have been shortened from the type of threads that are used on bolts (screw threads). Joints and such are "bolted" together. However bolts have threads, screw threads. The term "screw" is simply a derivation of a bolt with screw threads. Most US bolts have Unified Standard screw threads that follow that spec.
I Could Not See "mech Handbook"due To Unavailability.but Could See Encarta.
What Could I Understand Is
All The Cones And Cylinders Having Screw Threads Are Screws.however The Screws Used With Machines Are Called Machine Screws Or "bolts".the Threads Of Bolts Are Matched By The Threads On The Inside Of A Nut
Moreover---marine Propellers Are Also Termed As Screws And Aircraft Propellers Are Termed As Airscrews.--the Propeller Is Essentially A Screwthat,when Turned, Pulls Itself Through The Air Or Water In The Same Way That A Bolt Pulls Itself Through A Nut.--typical Propellers Consists Of Two ,three,four Blades ,each Of Which Is A Section Of A Helix,which Is The Geometric Form Of A Screw Thread.