According to searchsecurity.com:
"In computers, footprinting is the process of accumulating data regarding a specific network environment, usually for the purpose of finding ways to intrude into the environment. Footprinting can reveal system vulnerabilities and improve the ease with which they can be exploited."

Details of network footprinting methods can be found at http://searchwindowssecurity.techtarget.com/generic/0,295582,sid45_gci1115243,00.html:

"The first step in attacking any network is to figure out what to attack -- to develop a "footprint" of the target network. There are many techniques for this. For a full discussion, see one of the excellent books in the "Hacking Exposed" series. The basic goal is to learn more about the network. There is a lot to discover, including, but not limited to, the following:

• Network address ranges
  Host names
  Exposed hosts
  Applications exposed on those hosts
  OS and application version information
  Patch state of the host and the applications
  Structure of the applications and back-end severs
  Implementation details the sys admin posted to newsgroups or told a reporter about

After finding basic information such as this, you can use what you have to search out even more useful knowledge. For instance, once you know the IP address of the main server, you can utilize a number of websites in order to find detailed information such as the address of that IP. You can easily find out who manages the server, and once you have a name, can get their home address off any phone directory web page.

Footprinting is extremely hard for a company to protect against since any amount of public knowledge can be used to aquire much more detailed information.