What is a software error? 
A mismatch between the program and its specification is an error in the Program if and only if the specification exists and is correct.
Example: -
* The date on the report title is wrong
* The system hangs if more than 20 users try to commit at the same time
* The user interface is not standard across programs

Categories of Software errors 
* User Interface errors
* Functionality errors
* Performance errors
* Output errors
* Documentation errors

What Do You Do When You Find a Defect? 
IF A Defect is FOUND,
* alert the developers that a defect exists
* show them how to reproduce the defect
* ensure that if the developer fixes the defect it is fixed correctly and the fix
* didn't break anything else
* keep management apprised of the outstanding defects and correction trends

Defect Writing Tips 
Ideally you should be able to write defect report clearly enough for a developer to reproduce and fix the problem, and another QA engineer to verify the fix without them having to go back to you, the author, for more information.
To write a fully effective report you must :-
* Explain how to reproduce the problem
* Analyze the error so you can describe it in a minimum number of steps
* Write a report that is complete and easy to understand