Form API

Compact forms are always good to have and in-field labels is one of the feature that makes the form more compact and sleak. Conventionally the labels are set as "above" or "in-line" with the fields where as in-field labels are set with in the Label's respective fields. Technically form item or element fields are overlaid with their repective labels.

Inside Drupal APIs we can find separate set of APIs that deals with Forms, their elements and properties. Forms API reference guide has all the elements, properties and complete reference to the forms control structure where you can find easily a what properties all forms element carry. For more details please see Forms API Quick Start Guide.