From Unofficial BOINC Wiki
The BOINC System classifies the possible messages that the Participant might see, and it is useful to have a categorization of the possible types of messages and how important are they to the operation of the BOINC Client Software. For example, messages that just give you information that is "nice-to-know", but if you missed the message, well, "who cares". The basic classification scheme uses three classes of messages, and I added a fourth called "Status". A Status Message is one that gives information about the operational status of the BOINC Manager as it is running.
 Message Type Definitions
 Information Message
A Message that simply informs the Participant about something. An example is the message that is output as a result of the running of the Benchmark process that tells the Participant the number of CPUs that the BOINC Manager "sees".
 Status Message
This is a type of Information Message that the BOINC Manager outputs that informs you about the actual operational status of the BOINC Daemon as it exists "right now!" An example of this type of message would include those that are telling you that the Benchmarks are being run, or that a particular Work Unit is being processed, or has just been suspended and it is still in memory, etc. These messages, are of use in the monitoring of the BOINC Client Software and what it is doing. Once again, this is a type of Information Message, but one that gives you insight into the operation of the software.
If you happen to miss one of these messages, it still does not matter a whole lot, but they are something that we all look at to know what is going on.
 Warning Message
Now we get serious! Warning Messages are those messages that indicate that there is an operational issue with the BOINC Manager or BOINC Daemon as it is running. The operation of the software is not compromised, and whatever problem occurred, the BOINC Software was able to hang in there and continue processing and more than likely no harm was done.
Warning Messages are something that you do want to keep an eye on, and if they are persistent may indicate a problem with your computer, the Project's Science Application, or the BOINC Client Software itself.
An example of this type of message would be the one telling you that it is unable to contact the Scheduling Server for a particular Project. This is a "normal", and expected, condition; but if it happens all the time, well, there may be a problem somewhere â€¦
 Error Message
Oh, gosh, darn it! We have a significant issue. This is something serious and the BOINC Client Software was not able to handle the problem gracefully, or in a way that did not compromise the operation of the BOINC Client Software. This may range from the loss of a Work Unit, to the loss of a Result or Result Data File.