This tutorial shows how to create a Debian/Ubuntu mirror for your local network with the tool apt-mirror. Having a local Debian/Ubuntu mirror is good if you have to install multiple systems in your local network because then all needed packages can be downloaded over the fast LAN connection, thus saving your internet bandwidth.
Read it at Howtoforge
I want to say first that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!