An ISO 9001 audit

Answer
First, what is an audit?
It is a systematic, independent (to ensure objectivity) and documented process to get audit evidences and compare them with the audit criteria to conclude about the extension these criteria are fulfilled.
I use this image to explain what an audit is:

An auditor goes into reality, collects audit evidences (facts) and compares them with the audit criteria (the rules).
Each comparison is an audit finding.
Comparing the audit objective with the sum of all audit findings the auditor can draw a conclusion.
Second, what types of audits can we talk about?
There are three types of audits: first, second- and third-party audits. In a first party audit the audit manager belongs to the organization being audited. In a second party audit the audit manager belongs to a customer. In a third party audit the audit manager belongs to a certification body.
Third, what is an ISO 9001 audit?
An iso 9001 audit is an audit where the audit criteria is based on ISO 9001, a quality management standard.

The following material will provide you more information about audits:
Article - First-, Second- & Third-Party Audits, what are the differences? - https://advisera.com/9001academy/blog/2015/02/24/first-second-third-party-audits-differences/
Free webinar – How to perform an ISO 9001:2015 internal audit - https://advisera.com/9001academy/webinar/how-to-perform-an-iso-9001-2015-internal-audit-free-webinar-on-demand/
- Enroll for free course - ISO 9001:2015 Foundations Course - https://advisera.com/training/iso-9001-foundations-course/
- Enroll for free course - ISO 9001:2015 Internal Audit Course - https://advisera.com/training/iso-9001-internal-auditor-course/
- ISO 9001:2015 Documentation Toolkit - https://advisera.com/9001academy/iso-9001-documentation-toolkit/
- Book - ISO Internal Audit: A Plain English Guide - https://advisera.com/books/iso-internal-audit-plain-english-guide/