Domain names that end in .com are the “default” extension, which means that people tend to assume that your domain will end that way. These extensions also tend to be ranked higher by search engines making them easier for customers to find.
A site traffic ranking tool that uses a sampling approach where data is gathered from alexa toolbar users to rank websites by estimated volume of traffic. Caution should be taken when using Alexa data as the basis of decisions as the data is sampling based and Alexa can group sites.
GTLD - Generic Top Level Domain
Domain extensions (tld's) that are not linked to a specific geographic region and were originally intended to represent a specific usage e.g. .com (originally intended for commercial) and .biz (intended for business usage).
URL - Uniform Resource Locator
The address of a resource on the internet - usually used in connection with a full website address like http://www.domain.com/docs/page.html. This includes the protocol (eg http://), the domain name (eg domain.com) and possibly a page name and path to it(eg /docs/page.html).
An advertising program operated by Google whereby the ads displayed on a content site are automatically generated to match the content and revenue is shared with the site owner on a pay per click basis.