Different types of bolts,screws and nuts

BOLT:
An externally threaded fastner, designed for insertion through holes in assembled parts is called a bolt. It is usually intended to be used along a nut.

SCREW:
An externally threaded fastner capable of being inserted into the holes inside assembled parts (now comes the difference) of mating with a pre formed internal thread or forming it's own thread is called a screw. It has an ability of being tightened and released by torquing the head.

STUD:
A metal rod or shaft with threads on both sides is called a stud. Usually the rod is long, but depending upon the requirement the size can be changed. Given below is an image of a stud