"Installers" are typically scipts that install stuff. Debian packages (*.deb) are simply files that are compressed with a collection of files that go into specific places in the file system--this is the definition of "installed." Apt is simply a framework or a front-end for handling Debian packages.
There is an order to user-friendliness:
They all do the same thing. Open a terminal:
Practice using each method above, and you will quickly see the differences.
sudo apt-get moo