[CentOS] how to create a local update server.

could someone please point me in the right direction for creating a 
local repository for centos updates.

Scenario:  I have one centos server connected to the internet, it 
provides ntpd, httpd, and other services to both internal and external 
clients (only httpd is serving external clients).

the clients are connected to the first centos server and it functions 
as a NAT router for internet sharing purposes.

I'd like to create a local updates repository so that clients only need 
to check MY update server instead of the public ones.  I'd like the 
option of just being a repository of the "update list" (the list of 
packages that are updated), and being a repository of the .rpm files 
themselves.