The Field API allows custom data fields to be attached to Drupal entities and takes care of storing, loading, editing, and rendering field data. Any entity type (node, user, etc.) can use the Field API to make itself "fieldable" and thus allow fields to be attached to it. Other modules can provide a user interface for managing custom fields via a web browser as well as a wide and flexible variety of data type, form element, and display format capabilities.
The Field API defines two primary data structures, Field and Instance, and the concept of a Bundle. A Field defines a particular type of data that can be attached to entities. A Field Instance is a Field attached to a single Bundle. A Bundle is a set of fields that are treated as a group by the Field Attach API and is related to a single fieldable entity type.
For example, suppose a site administrator wants Article nodes to have a subtitle and photo. Using the Field API or Field UI module, the administrator creates a field named ‘subtitle’ of type ‘text’ and a field named ‘photo’ of type ‘image’. The administrator (again, via a UI) creates two Field Instances, one attaching the field ‘subtitle’ to the ‘node’ bundle ‘article’ and one attaching the field ‘photo’ to the ‘node’ bundle ‘article’. When the node system uses the Field Attach API to load all fields for an Article node, it passes the node’s entity type (which is ‘node’) and content type (which is ‘article’) as the node’s bundle. field_attach_load() then loads the ‘subtitle’ and ‘photo’ fields because they are both attached to the ‘node’ bundle ‘article’.
Field definitions are represented as an array of key/value pairs.