Adwords and DoubleClick essentially share the same DNA there are many differences that set the two apart. Adwords will always be the standard when it comes to online marketing on the Google network, and works effectively on any account no matter the size or budget of the client. DoubleClick offers industry leading tools suited for large accounts that require visibility across all ad networks and advanced marketing strategies, and may not be suited for all clients.
You can try to search the net or read one of the content at The Image Media blogsite, that is wear I get this definition that will help you understand the difference between the two.