What Information We Add in Name Server and Why we Associate NS When we Purchase a Domain

In Name Server (along with other info, the most important is) we define a mapping of Our Domain/Sub-Domain Name and Its Hosting Server IP Address

What other info? Its usually include following things: The email server address... called MX records. Defining domain aliases. Configuring it to redirect to other domain. Adding some info provided by third parties to verify that we own the domain. Sometime we want our particular sub-domain to link to another DNS, who better knows the hosting server IP. we can also do that e.g. when we configure our domain to Blogger free sub-domains, we map our domain to google name server. etc. The rest is done by Google in their NS. But you don't need to bother all these details as far understanding basics is concern.