two or more molecules having same molecular formula but different physical and chemical properties are called isomers .this phenomenon is called isomerism.
ex:C4H10 , HAS 2 ISOMERS.THEY ARE BUTANE AND 2-METHYL PROPANE.
there are 2 types of isomerism .they are i)structural isomerism  ii)stereoisomerism
structural isomerism  is of 5 types : 1) chain 2)poistional 3)functional 4)metamerism  5)tautomerism.
stereoisomerism is of 2 types : 1) configurational  2)conformational
						

							
The existence of two or more compounds with same molecular formula but different properties (physical, chemical or both) is known as isomerism; and the compounds themselves are called isomers. The term was given by Berzelius. The difference in properties of two isomers is due to the difference in the arrangement of atoms within their molecules.
						

							
it generally means difference in properties of 2 compounds having same molecular formula
                 due to difference in their structure.
it is of 2 types:-
                     1.structural isomerism :- molecules having same molecular formula but different structure.
                     2.stereio isomerism :-molecules having same molecular formula but different spatial arrangement.
						

							
Two or more compound having same molecular formula but different  structural Formulas are called isomers and process is called  Isomerism..For Example: 
 Butane(C4H10) has two isomers.
 CH3-CH2-CH2-CH3.
 its has two types :
 1)Structural Isomersim
 2)Cis-trans Isomersm(Geometric Isomersm)