WordPress’s “Text” Format

Text its just plain text, just as its saved in your db.

WordPress changes line breaks with paragraphs with the function wpautop, through the filter the_content and the_excerpt.

If you need to remove the wpautop behavior, you can remove the filter by doing this in your theme’s functions.php:

remove_filter( 'the_content', 'wpautop' );
remove_filter( 'the_excerpt', 'wpautop' );

And maybe adding your own customized function:

add_filter( 'the_content', 'my_custom_format' );
add_filter( 'the_excerpt', 'my_custom_format' );

function my_custom_format ($text) {
  // do something with $text
  return $text;
}