You can use the -i option with the dpkg command to install a downloaded package:
root@ubuntu-linux:/tmp# dpkg -i cmatrix_1.2a-5build3_amd64.deb
Selecting previously unselected package cmatrix.
(Reading database ... 178209 files and directories currently installed.) Preparing to unpack cmatrix_1.2a-5build3_amd64.deb ...
Unpacking cmatrix (1.2a-5build3) ...
Setting up cmatrix (1.2a-5build3) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
root@ubuntu-linux:/tmp#
And that's it! Now run the cmatrix command:
root@ubuntu-linux:/tmp# cmatrix
You will see the matrix running on your terminal like in the following image:
Figure 2: cmatrix
We have taken the long way to install the cmatrix package. We first downloaded the package, and then we installed it. You can install a package right away (without downloading it) by running the command apt-get install followed by the package name:
apt-get install package_name
For example, you can install the GNOME Chess game...