Set Theory starts very simply: it examines whether an object belongs, or does not
belong, to a set of objects which has been described in some non-ambiguous way.
From this simple beginning, an increasingly complex (and useful!) series of ideas

can be developed, which lead to notations and techniques with many varied

applications. A set can be defined as a collection of things that are brought

together because they obey a certain rule.These 'things' may be anything you
numbers, people, shapes, cities, bits of text ..., literally anything.The key fact

about the 'rule' they all obey is that it must be well-defined 

hope it helps............. :)

good explanation
now i knew why there are many thanks to you :)
1  the collection of all  boys in your class  =   set .
2  the team of eleven best  cricket batsmen  of the world = not set.
  for   example.
natural number, prime number  quadrilaterals in a  plane etc.  all   example  see  so  far  are well defined  collection of objects or  ideas. a well defined collection of onjects or ideas is known  as a set.  set  theory is a comparitively new concept in mathematics..

all the objects  in the set should have a common feature or property...

hope its help u..