An auditor is a person who conducts an audit. (I say that first, because an audit need not necessarily be conducted by an auditor, it is possible to create an automated audit process. Also, 'non-auditors' may, or may be required to, participate in 'an audit'.)
An audit is a process in which checks are made to ensure that there is a reasonable level of evidence to demonstrate satisfactory compliance to a defined procedure, or set of procedures.
You cannot audit a process if it has no defined procedure. You cannot audit a process if there is no evidence of the procedure. An 'audit-able' procedure must therefore include steps within it in which evidence is recorded.
The audit should reveal compliances and non-compliances in the particular areas/topics of the procedure(s) examined, and will generally also make observations in regards matters that are either particular strengths or weaknesses of the practices being adopted in regards the essential purpose of those practices.
Audits are generally 'themed' so as to 'sample' the procedure, rather than look at the whole. For example, an audit may follow a particular job through, or may examine one particular step of a procedure across different jobs, or look at a particular piece of kit.
The audit is therefore unlikely to reveal all non-compliances - this is not [necessarily] an audit's purpose. The audit is an examination of specimen matters. The purpose is to ensure the procedures are generally operating as required, and those procedures should have embedded within them the means to correct and self-correct issues. A good audit might raise areas of the procedure that are not 'self-checking', so to speak. It is the procedure that should correct itself, it is not for the audit to correct a procedure.