If you have big web site and want to serve lot of advertisements, you need to have an advertising server.

There are free open source advertising server like openx, which is PHP, MySQL script, that can be used to run your own adverting server.

http://www.openx.org

There are professional hosted solution available from following providers

http://www.doubleclick.com
http://www.aquantive.com
http://www.247realmedia.com
http://www.accipiter.com
http://www.mediaplex.com
http://www.valuead.com
http://www.zedo.com
http://www.falkag.com
http://www.bluestreak.com
http://www.checkm8.com