Domain registrars are an authority that can assign domain names directly under one or top-level domains. These domains are registered with InterNIC, a service of ICANN, which enforces the uniqueness of domain names across the Internet. Each domain name becomes registered in a central database known as the WholS database.

What is the name of the database where the domain name is registered?

WhoIS database

Popular domain registrations include eg Amazon, Flipkart, GoDaddy.com

Top Level Domain names like .com, .edu, .gov

This top-level domain name is controlled by the Internet Assigned Number Authority (IANA) in a root zone database which is essentially a database of all available top-level domains. You can view this database by visiting:

http://www.iana.com/domains/root/db