Domain names end with extensions called TLDs. The TLD is the letters that appear after the last dot in the domain name. For example, .com is a TLD. Fortunately there are a wide variety of TLDs available, and there are even geographically specific TLDs that specify the location of a web site. The cost of a domain name will usually depend upon the TLD.
Dot com domains are the most common, with .org, .net and .info following closely behind in terms of popularity.