The list of syslog severity Levels:

0 Emergency: system is unusable
1 Alert: action must be taken immediately
2 Critical: critical conditions
3 Error: error conditions
4 Warning: warning conditions
5 Notice: normal but significant condition
6 Informational: informational messages
7 Debug: debug-level messages

Recommended practice is to use the Notice or Informational level for normal messages.

A detailed explanation of the severity Levels:

DEBUG:
Info useful to developers for debugging the application, not useful during operations

INFORMATIONAL:
Normal operational messages - may be harvested for reporting, measuring throughput, etc - no action required

NOTICE:
Events that are unusual but not error conditions - might be summarized in an email to developers or admins to spot potential problems - no immediate action required

WARNING:
Warning messages - not an error, but indication that an error will occur if action is not taken, e.g. file system 85% full - each item must be resolved within a given time

ERROR:
Non-urgent failures - these should be relayed to developers or admins; each item must be resolved within a given time

ALERT:
Should be corrected immediately - notify staff who can fix the problem - example is loss of backup ISP connection

CRITICAL:
Should be corrected immediately, but indicates failure in a primary system - fix CRITICAL problems before ALERT - example is loss of primary ISP connection

EMERGENCY:
A "panic" condition - notify all tech staff on call? (earthquake? tornado?) - affects multiple apps/servers/sites...