How can I able to find the usage of default tables available in drupal.
Is there any documentation available?
For example: there is a table called node. I need to know what is the usage of it and how it acts.
Any suggestions or answers will be helpful and grateful.
Your question is not very clear (the term “usage” is quite ambiguous), but you could install the Devel module. After setting it up it will show, for every page loaded (home page included), which SQL queries are run.