Diffrerence between attended and un attended installation?

An unattended installation has no permanent staff.

With computer software installations an attended installation means that the user is watching the installation and is expected to answer questions that come up during the install.

An unattended installation can be started by a user and then not watched because the answers to any questions from the install have been predetermined and stored in something called an 'answer file'. The answers are taken from that file and the user is not required to sit and wait during the installation to answer any questions.

This is a common technique with Windows OS installs.