Strangely enough if you go to Google and enter "What are nodes in Drupal" it tells you the following:
All content on a Drupal website is stored and treated as "nodes". A node is any piece of individual content, such as a page, poll, article, forum topic, or a blog entry. Comments are not stored as nodes but are always connected to one. Treating all content as nodes allows the flexibility to create new types of content.
Even more strange is that in another post by you it says "We have good experience in design and development of websites." So why didn't you already know this?
Why you felt the need to ask this question instead of looking it up for yourself is beyond me. Did you realy want to know? If you did it's 10 times quicker to look it up. If you didn't really want to know then you're just building up your post count by posting useless threads.
And nodes are Entities within Drupal 7 and Drupal 8. Entities are agnostic pieces of content within Drupal. The Node is the most widely known Entity in Drupal.
Node is an entity hold in Drupal core. We can create unlimited number of bundles under node entity taxonomy, comment, etc are other entities present in drupal core. D7 and D6 has two default bundles. D7 - article and basic_page. D6 - story (same as article) page (). ie Default bundles for a node in Drupal 7 installatin are article and basic_page. In D6 its (default) story and page