NS Critical vs Non-Critical errors

Can anyone give me more or less an idea of what errors generated in the Altiris Log Viewer is defined as System Critical errors and what are minor processing errors?

Example: If i had to tell a new Techinician to go through the logs and check errors in my log, which ones do i need to be worried about and which ones are not so critical to system performance. Or is it simply down to experience to help you adentify the critical errors?