When you install WordPress, it will come with a default theme to show the front-end of your website. You can change the theme at any time and are not obligated to use the default theme that came with your installation.
The default theme’s that come with WordPress are designed to specifically showcase all of the current features of WordPress. This means that these themes are normally feature intensive and can be used to create a wide variety of basic websites. Even if you decide to install another theme, do not delete your default theme. It is your safety net in case something happens to your primary theme and it gets deleted or destroyed.
When WordPress was released back in 2003, users did not have the ability to change their themes. That all changed when the Kubrick theme came out with the WordPress 1.5 update. This gave WordPress users the ability to change their theme. Kubrick was the default theme until 2010, when WordPress 3.0 was released. WordPress 3.0 came with the TwentyTen default theme. Since then, the WordPress team has set out to create a new default theme to represent each new year, however, TwentyTen is still one of the most popular and iconic themes in association to total downloads.