ROS supports the following logging levels (in increasing order of relevance):

• DEBUG
• INFO
• WARN
• ERROR
• FATAL

These levels are part of the function used to output messages, so it's something you can easily experiment with, simply by using the syntax below:

ROS_<LEVEL>[_<OTHER>] 

Each message is printed with a particular color. The colors are as follows:

DEBUG in green 
INFO in white 
WARN in yellow 
ERROR in red 
FATAL in purple 

Each message level is meant to be used for a different purpose. Here, we suggest the uses for each of the levels:

• DEBUG: Use them only when debugging; this information should not be displayed in a deployed application, as it's only for testing purposes
• INFO: These should be standard messages to indicate significant steps or what the node is doing
• WARN: As the name suggests, use them to provide a warning that something might be wrong, missed, or abnormal, but that the application can still
  run regardless
• ERROR: These can be used to indicate errors, although the node can still recover from them; however, they set a certain expectation regarding the node's behavior
• FATAL: These messages usually expose errors that prevent the node from continuing its operation