What is the difference between SAS AU and Auditing standard AU?

The AU's are the Generally Accepted Auditing Standards as promulgated by the ASB (a committee of the AICPA). They are numbered in sections:

AU 100s - Introduction

AU 200s - General standards

AU 300 - Standards of Fields work

and so on

The Statements on Auditing Standards (SAS) are also issued by the ASB. They are statements made by the ASB that help auditors understand how to meet the requirements of the AUs and are they are numbered consecutively as the pronouncements are issued. So, for example, SAS 106 pertains to audit evidence. Audit evidence falls under standards of field work so SAS 106 is incorporated into the 300 section of the AUs, in Section 326 to be specific. There is more specific guidance in the form of interpretation that is AU 9326. When you see an AU that starts with a 9 it is an interpretation of the same number AU.