Just days after Verisign launched the Site Finder Service, ICANN wrote a strongly worded letter ordering them to disable it. Verisign complied, but not without writing a strongly worded letter of their own, in the form of a lawsuit. Verisign argued that ICANN overstepped their bounds in trying to control their activities, and ICANN argued… that Verisign overstepped their bounds in performing those activities.
For the most part, you can pick whichever TLD you prefer, though a few have restrictions -- you can't buy a .edu domain unless you run an accredited post-secondary school. Many, but not all, country-specific domains (.us, .uk, .jp) are restricted to citizens of the country in question. Unrestricted country domains are often used for "domain hacks," where the extension becomes part of the title, such as YouTube's alternate address, "youtu.be."
Domain names can be used in a number of ways. Direct visitors to your website’s home page or use complementary domains to send them to specific areas of your website ( e.g. yourcompany.jobs for a careers page). Or, forward a custom domain to an existing social media account, like your Twitter profile or Periscope channel. With a domain name you can even set up custom email addresses like email@example.com, which can be used to conduct business or communicate with your followers. More.