What is the main difference between forensic accounting and auditing?

Auditing is specifically checking to see if the accounts and/or systems conform to a pre-defined set of specifications (GAAP in this example, but there are all sorts of audits).

Forensic Accounting is investigative. Instead of seeking conformance, you are looking for outliers or specific patterns. Generally I've seen this in support of some sort of legal process. The outcome is less general (an audit report), and is generally useful as evidence in court with regard to certain occurances (where did my cash go; why did this customer get 10% discounts; why am I paying 3x the going rate for sand; etc). Note it is a done-thing to use FA outside of a legal context, so it isn't just constrained there.

Auditing is designed to provide an overall opinion on financial reporting in some form, but limited review for particular issues, particularly "negative" issues, and do not involve detailed specific analyses.

Forensic accounting is much more detailed and focused, particularly to fully understand a specific issue or family of issues, and often used in resolving disputes, including litigation support.