A WordPress theme is basically the skin of your blog. It defines all those bells and whistles you want to stand out of the competition.
WordPress default theme is quite basic and bloggers usually replaced the theme with other free or paid themes which have more options to glorify blog’s look.
A WordPress theme is essentially a collection of customized layouts, scripts and stylesheets to change or enhance the look and feel of a WordPress website. WordPress themes directory has a huge collection of free WordPress themes which could be added to any WordPress website.
Go to Appearance > Themes, here you can see all installed themes.
To install a new theme in WordPress, click Add New at the top. From the next window, you can see a free list of WordPress themes which you can select any of them or click the Upload Theme to upload custom WordPress theme and use.
Once the theme is installed, just Activate it and your blog just got a new look with the selected theme.
There are customized WordPress themes also available on internet which are specially designed for a specific niche. You will see photography themes, restaurant themes, life style, kids, woman, spa and hundreds of options for almost every niche.