Internationalization

Return the translation of its argument

__( 'Some text', 'textdomain' );

Output the translation of its argument

_e( 'Some text', 'textdomain' );

Display translation of text with variable

printf( __( 'Some text %s', 'textdomain' ), $variable );

Store translation of text with variable

$text = sprintf( 'Some text %s', 'textdomain' ), $variable );