While testing when a tester executes the test cases he might observe that the actual test results do not match from the expected results. The variation in the expected and actual results is known as defects. Different organizations have different names to describe this variation, commonly defects are also known as bug, problem, incidents or issues.
Every incident that occurs during testing may not be a defect or bug. An incident is any situation in which the software system has a questionable behavior, however we call the incident a defect or bug only if the Root Cause is the problem in the tested component.
Incidents can also occur by some other factors as well like testers mistake in test setup, environment error, invalid expected results etc.
We log these incidents just to keep track of the record of what is observed during the testing so that we can find out the solution to correct it.