A WordPress Theme is a collection of files that work together to produce a graphical interface with an underlying unifying design for a weblog. These files are called template files. A Theme modifies the way the site is displayed, without modifying the underlying software. Themes may include customized template files, image files , style sheets , custom Pages, as well as any necessary code files . For an introduction to template files,

Using Themes
WordPress comes with two themes; the default Twenty Thirteen theme and the previous default, Twenty Twelve theme. You can switch between Themes using the admin panel. Themes that you add to the theme directory will appear in the Administration Panels > Design > Themes as additional selections.

Selecting the Active Theme

Log in to the WordPress Administration Panels.
Select the Appearance panel, then Themes.
From the Themes panel, roll over the Theme thumbnail image for the Theme you are interested in to see options for that theme.
You can view more information about any theme by clicking Theme Details.
A live preview of any Theme (using your blog's content) can be seen by clicking Live Preview.
To activate the Theme click the Activate button.

Your selection will immediately become active.