ATDD is also called Behavior Driven Development

ATDD or Acceptance Test Driven Development is often referred to as BDD or Behaviour Driven Development.

This is the practice of writing high level acceptance tests, understandable by business and customer level, in satisfying that a piece of work does what it is supposed too. Focusing on the behaviour of the system from the customer or users perspective, allows the project to progress from a functionality and behaviour perspective, so that once the system does what it's meant to do its complete.

Acceptance tests will usually be ran at the project level, where it is possible to get an overview of the state of a project by running these, and see what percentage pass, determines what percentage of the project is complete.  Inline with these, developers may use separate Unit Tests inline with each acceptance test focusing on the units of code that make up an acceptance milestone.