You can incorporate data in your maps from layers that have been published to a public web server. Web-based data is published using one of two open standards developed by the OpenGIS Consortium (OGC):
The number of Web Map Services (WMS) and Web Feature Services (WFS) that implement OpenGIS interfaces on the Internet is increasing all the time, as more organizations adopt the open standards. At the moment, there is a lot more data available in WMS format than in WFS.
If you find a server that provides WMS or WFS data that interests you, you can browse the content of the published layers using viewer software, such as the Gaia viewer shown in the illustration below.
Once you have located some WFS or WMS data, you need to get the URL of the page that serves the published layers. Often, this is not a standard web page that you can open in a browser, but a page that has been programmed using a scripting language such as CGI, PHP, or ASP. A typical web server address looks like this: http://wms.jpl.nasa.gov/wms.cgi