An IT audit is a standalone, paid, consulting engagment. A proper audit involves an interview with your IT stakeholders, an understanding of your business (operations and goals), and time to perform a hands-on discovery of your entire IT environment. You should walk away from an IT audit with comprehensive documentation about how your IT works today, and an outline and budget of the IT initiatives needed to address your risks and support the strategic goals of your business.
An IT assessment on the otherhand is a quick, often automated, partial exploration of your IT environment. There are many cases where an IT assessment makes sense to do, maybe you just want a list of your internet-facing services. Do not confuse an IT assessment with an audit, an assessment won’t give you anywhere near the level of detail and guidance that a true IT audit will provide.