In case you need to export your contacts from other mail providers or programs, please refer to these links that can provide some information to you. You may also want to check the  help section of your email program to find more information about it:

http://email.about.com/od/gmailtips/qt/Import_Addresses_from_Hotmail_Outlook_Yahoo_Others_in_Gmail.htm

http://mail.google.com/support/bin/answer.py?answer=12118