Heterocyclic compounds, unlike homocyclic compounds, are the chemical compounds that contain atoms of at least two different elements in their ring structures. Most of the homocyclic compounds (compounds that have atoms of only one element in their rings) can also be converted to heterocyclic compounds by replacing one or more atoms with that of another element. oxygen, nitrogen and sulfur form most of the heterocyclic compounds, by replacing one or more carbon atoms with that of any of these elements.
Some examples of heterocyclic compounds are pyridine, pyrimidine, dioxane, furan, thiazole, pyran, etc. Some of the most studied heterocyclic compounds are nucleobases that make up our DNA. These include cytosine, guanine, adenine, thymine and uracil. There are many other examples of these compounds and you can go through the attached link for more information.
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In order to facilitate the study of large number of organic compounds , they have been classified into a few series or families based on the basis of similar properties .All the known organic compounds have been divided into two main types:
1. Open chain or Aliphatic compounds
2. Closed chain or Cyclic compounds : Organic compounds with a closed chain of atoms are termed as closed chain or cyclic compounds.
These are further sub-divide into two classes :
a) Homocyclic or Carbocyclic : These are the compounds having a ring(monocyclic) or rings(polycyclic) of carbon atoms only in the molecule.
b) Heterocyclic : These are cyclic compounds having ring or rings built up of more than one kind of atoms. The most common hetero-atoms besides carbon are Oxygen , Nitrogen and Sulfur.Few examples of this class are Furan , Pyrrole , Thiophene.