If you are familiar with the Drupal forms API you know the concept of form elements as the basic building blocks of any form. Drupal provides several types of elements, some rather straightforward (#type textfield), some more advanced (#type password_confirm).
It is also possible to define custom form elements that enable you to implement complex widgets with ease.
At the moment, the focus of Elements is the tableselect widget. It provides a clean, easy way to create tables with one checkbox per row, multi-row selection and a select all checkbox, without making your code a complicated mess.