15. Explain the concept of node in drupal.

A node in Drupal is the generic term for a piece of content on your web site. (Note that the choice of the word "node" is not meant in the mathematical sense as part of a network.) Some examples of nodes:
• Pages in books
• Discussion topics in forums
• Entries in blogs
• News article stories
Each node on your site has a Content Type. It also has a Node ID, a Title, a creation date, an author (a user on the site), a Body (which may be ignored/omitted for some content types), and some other properties. By using modules such as the contributed Content Construction Kit (CCK) module, the core Taxonomy module, and the contributed Location module, you can add fields and other properties to your nodes.

Added Further ::

Nodes: The secret to Drupal's flexibility

We don't talk about "nodes" every day, but since they are at the heart of Drupal's design, they deserve further investigation. At its most basic, a node is a set of related information. When you create a new blog post, you are not only defining its body text, but also its title, content, author link, creation date, taxonomy (tags), etc. Some of these elements will be shown by the theme layer when the node is displayed. Others are meta-data that control when the node will show up at all - such as taxonomy or publishing status.

Since each item of content is stored as a node, and contains the same basic information, each can be handled in a standard way by both Drupal core and contributed modules. This allows site builders to choose exactly where they want content to show up, and exactly how they want it to look in each case. Most of a Drupal site builder's time is spent defining what kinds of information you want to store in your nodes, and configuring the structures (menus, taxonomy trees, views, panels) in which to display them.

As suggested before, you aren't limited to a single way of presenting your site's content. You can define as many navigation schemes, custom themes ("skins" for the site), blocks (small bits of content, such as the five most recent blog articles described earlier), and feature sets as there are distinct audiences for your site.

Read further http://drupal.org/getting-started/before/overview