According to ISO 9000:2015 an audit is 

"A systematic, independent and documented process for obtaining objective evidence and evaluating it objectively to determine the extent to which the audit criteria are fulfilled" 

I don't find that a particularly useful definition (does anybody?) so I prefer to talk about an audit (and the associated report) telling the story of a process - it might be a story following an order from initial customer enquiry to shipping it out of the door, or a story about the management and leadership processes controlling and directing the overall direction of a company.

A good audit gets under the skin of a company, finds out what is actually going on and identifies any non-conformities or opportunities for improvement. It also provides an opportunity for a range of people to be involved in the process and prompts conversations about the management system and culture of the company. A bad audit stays on the surface of a process, doesn't dig deeply into what is happening and is a bland record of what has been checked off. 

A good audit report is a thorough, in depth review of what evidence was seen (using photos, screenshots, interviews, descriptions) and what audit trails were followed up, as well as clearly recording the findings and audit conclusions. A bad audit report is generally much shorter, with less detail and non-verifiable information or opinions - it may even involve a checklist of x's in boxes (my pet hate!)  

A good audit report helps everyone understand the business better and builds commitment to the management system - a bad one makes everybody involved feel a bit depressed and unenthusiastic about the whole process.