The file functions.php is one of the files that must be found in a WordPress theme. Unlike other files, its role is not to display content but to add functionality to the theme. In a sense, it acts as a plugin to the theme.
If you modify a functions.php to add features to your theme, you should know that if you change the theme, these features will be lost or will have to be added in the new theme.
Here are some examples of features you can add to your site using the functions.php :
- Add, modify or delete custom content types (portfolio, services, …);
- Add, modify or remove new taxonomies and link them to personalized content;
- Add a default Gravatar type;
- Disable the search function;
- Customize the length of an excerpt (it is 55 words by default);
- Disable RSS feeds;
- Display the number of subscribers to your Twitter account.
This list is far from being exhaustive. The customization possibilities are countless.