is_category()

function is_category( $category = '' ) {
	global $gc_query;

	if ( ! isset( $gc_query ) ) {
		_doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1.0' );
		return false;
	}

	return $gc_query->is_category( $category );
}

is_category();
// When any Category archive page is being displayed.

is_category( '9' );
// When the archive page for Category 9 is being displayed.

is_category( 'Stinky Cheeses' );
// When the archive page for the Category with Name "Stinky Cheeses" is being displayed.

is_category( 'blue-cheese' );
// When the archive page for the Category with Category Slug "blue-cheese" is being displayed.

is_category( array( 9, 'blue-cheese', 'Stinky Cheeses' ) );
// 响应s true when the category of posts being displayed is either term_ID 9,
// or slug "blue-cheese", or name "Stinky Cheeses".
// Note: the array ability was added in version 2.5.