apache2 package is used to select the other packages you need as part of the Apache installation.
apache2-mpm-prefork defines which 'processing model' will be used (
prefork in this case). The other options are
One is not necessarily more (or less) secure than another, as the differences between each are in how they handle setting up to receive new requests -
For most uses the
apache2-mpm-worker MPM is appropriate.
If you use
aptitude on your Debian system, when you select the
apache2.2-common will be selected for you automatically (along with any other support packages which are not already installed on your system). Similarly:
apt-get install apache2