Metadata

Overview
The Metadata API is a simple and standarized way for retrieving and manipulating metadata of various WordPress object types.
Metadata for an object is a represented by a simple key-value pair.
Objects may contain multiple metadata entries that share the same key and differ only in their value.
Function Reference
Add/Delete Metadata:
add_metadata()
delete_metadata()
Get/Update Metadata:
get_metadata()
update_metadata()
Database Requirements
This function assumes that a dedicated MySQL table exists for the 
$meta_type you specify. Some desired 
$meta_types do not come with pre-installed WordPress tables, and so they must be created manually.
Default Meta Tables
Assuming a prefix of 
wp_, WordPress’s included meta tables are:
wp_commentmeta: Metadata for specific comments.
wp_postmeta: Metadata for pages, posts, and all other post types.
wp_usermeta: Metadata for users.
Meta Table Structure
To store data for meta types not included in the above table list, a new table needs to be created. All meta tables require four columns.
meta_id – BIGINT(20): unsigned, auto_increment, not null, primary key.
object_id – BIGINT(20): unsigned, not null.
Replace object with the singular name of the content type being used.
For instance, this column might be named post_id or term_id.
Although this column is used like a foreign key, it should not be defined as one.
meta_key – VARCHAR(255): The key of your custom meta data.
meta_value – LONGTEXT: The value of your custom meta data.
Source File
Metadata API is located in 
wp-includes/meta.php.
Related
Metadata API: add_metadata(), get_metadata(), update_metadata(), delete_metadata().