Can we install .deb format on ubuntu 11.10 or 12.10 using sudo apt-get install???
In Linux, most software is installed through the package manager. This is an application that connects to a bunch of software repositories on the internet that host databases of .deb files (or .rpm in Red Hat variants). In the terminal (command-line window), you can install packages from the repositories via the command apt-get (or yum in Red Hat variants), which needs to be run as superuser. So, if you want to install GIMP (GNU Image Manipulation Program, i.e., a kind of open-source version of Photoshop), then you would open the terminal and write:
$ sudo apt-get install gimp
(note: the $ sign represents the prompt, you don't type that)
The sudo command means "superuser do". It allows you to execute one command (whatever follows it) with superuser privileges. It will ask you for your user password (same as when loging in).
The apt-get command is the program that can connect and fetch packages from the repositories. The option install speaks for itself. And the rest is a space-separated list of packages you wish to install (e.g., "gimp"). If you wish to remove a specific package (e.g., "gimp"), you would do:
$ sudo apt-get remove gimp
If you don't know the name of a package, you can always do a search that will return a list of suggestions:
$ sudo aptitude search gim (note: missing 'p', 'gimp' will be suggested)
For .deb files however, you have to go one step lower in the chain of commands. The actual package manager utility is called dpkg (or rpm for Red Hat variants). If you have downloaded a .deb file and want to install it, you can do:
$ sudo dpkg -i some_file.deb
which will install the package some_file.deb.
However, all this command-line interfacing is not necessary in Ubuntu or most other distributions. If you have a .deb file, you should be able to just go in the file explorer (Nautilus), and click / double-click on the .deb file and it should trigger the installation through a little GUI window that pops up. If you want to install any other software from the repositories, you can simply find, in the main menus, the application called "Software Center" or "Synaptic Package Manager" (depending on the version of Ubuntu). That application will allow you to do the same as what apt-get does, but with the convenience of a nice little GUI where you can search for apps, read descriptions of them, and install/remove at will.