http://www.debian.org/doc/manuals/apt-howto/
Installing packages
# apt-get install xchat
APT will search it's database for the most recent version of this package and will retrieve it from the corresponding archive as specified in sources.list. In the event that this package depends on another -- as is the case here -- APT will check the dependencies and install the needed packages. See this example:
# apt-get install nautilus
Reading Package Lists... Done
Building Dependency Tree... Done
The following extra packages will be installed:
bonobo libmedusa0 libnautilus0
The following NEW packages will be installed:
bonobo libmedusa0 libnautilus0 nautilus
0 packages upgraded, 4 newly installed, 0 to remove and 1 not upgraded.
Need to get 8329kB of archives. After unpacking 17.2MB will be used.
Do you want to continue? [Y/n]
The package nautilus depends on the shared libraries cited, therefore APT will get them from the archive. If you had specified the names of these libraries on the apt-get command line, APT would not have asked if you wanted to continue; it would automatically accept that you wanted to install all of those packages.
This means that APT only asks for confirmation when it needs to install packages which weren't specified on the command line.
The following options to apt-get may be useful:
-h This help text.
-d Download only - do NOT install or unpack archives
-f Attempt to continue if the integrity check fails
-s No-act. Perform ordering simulation
-y Assume Yes to all queries and do not prompt
-u Show a list of upgraded packages as well
Multiple packages may be selected for installation in one line. Files downloaded from the network are placed in the directory /var/cache/apt/archives for later installation.
You can specify packages to be removed on the same command line, as well. Just put a '-' immediately after the name of the package to be removed, like this:
# apt-get install nautilus gnome-panel-
Reading Package Lists... Done
Building Dependency Tree... Done
The following extra packages will be installed:
bonobo libmedusa0 libnautilus0
The following packages will be REMOVED:
gnome-applets gnome-panel gnome-panel-data gnome-session
The following NEW packages will be installed:
bonobo libmedusa0 libnautilus0 nautilus
0 packages upgraded, 4 newly installed, 4 to remove and 1 not upgraded.
Need to get 8329kB of archives. After unpacking 2594kB will be used.
Do you want to continue? [Y/n]