I'm probably not the best person to give an indepth description of Access lists, but having just passed my CCNA I think I can give you a heads up.
Access lists are basically used as a filter or a trigger on an network device interface. A filter meaning that they would stop or allow (lets just use tcp/ip) packets from moving through an interface based on source or destenation IP address or the TCP port they are using. They are used as a trigger to ( most simply ) to tell a router when to dial out its ISDN connection... IE. When an traffic that matches the condition of the ACL it dials out the ISDN line.
This is the really quick and dirty. What you would want to know for the CCNA is beyond the scope of a post here. Whatever CCNA book you are using should have a good chapter or more on it.
Here is a good site with a good basic breakdown of ACL's.
www.networkclue.com/routing/Cisco/access-lists/index.aspx
MY CCNA exam asked me to make a 3 line Access list that would prevent all traffic from 1 network to another, except 1 specific ip address that was to have HTTP (port 80) access in to one specific machine. So I would use that as your basis for what you need to know...
