This is a section of the aprs.fi user guide.
The message browsing page displays APRS chat messages sent between users. Initially it shows the most recent messages seen on the APRS-IS. If you look up a callsign, you will see all messages sent by or destined to that call. Messages sent by the station are shown with a red background, and received ones are shown in green.
aprs.fi currently does not have the possibility to transmit messages (or any other type of data), it serves only as a place to view data.
Status message packets are shown in the status message page. Initially, if opened without a callsign, it will let you browse the stations which have transmitted status messages, sorted by callsign. If you look up a callsign, you get to browse old status messages transmitted by that station over time.
Status messages are often confused with the comment text. They're another method to send some descriptive status message for the station, the difference from comments being:
2010-07-02 10:09:00 UTC: CT2XXX-11>APU25N,WIDE3-3,qAR,CT1AKV:>282308zUI-View32 V2.03
As you can see from this example, the timestamp is quite often invalid, so it is ignored by aprs.fi. In practice, you can put your descriptive message in the comment text or the status message. It is not wise to put the same text in both comment and status, as it will then be sent twice, which will place unnecessary load on the busy APRS network.
APRS bulletin messages are used for sending local announcements to other APRS network users. The aprs.fi Bulletin board page shows all bulletins which have been transmitted recently and passed on the APRS-IS. The page allows looking up custom bulletin groups using the links provided in the beginning of the page.
aprs.fi user guide