APRS-IS (Automatic Packet Reporting System-Internet Service) - is an Internet-based backbone network which interconnects APRS radio networks. The core network consists of three first-tier servers and numerous second-tier servers, providing a unified worldwide APRS stream to connected clients.
IGates connect the APRS-IS to radio networks, and typically consist of an Internet-connected PC, radio, and TNC.
There is a newer APRS-IS network called "APRS2" which offers more regionalised connectivity into the APRS-IS backbone, the list is available at: http://www.aprs2.net/
Once APRS packets make it through an igate to the internet system, they get recorded in various databases that are then searchable. This is an excellent way sometimes of examining and improving your station, assuming your packets are making it to an igate that is.
Here are a few of these sites: