The default user interface in UBUNTU since UBUNTU 11.04 (Oneiric Ocelot) is UNITY interface. The default gnome classic interface is removed from the default installation in recent versions. So, if you like the old, classic interface with 'Applications' menu on top left corner accessible with alt+f1 key, you would want to revert back to the gnome-classic interface. This article explains how you can return to classic interface from the default unity interface.
Install the required package
This is the UNITY interface available in UBUNTU
The gnome classic is not installed by default. The package required for classic gnome is 'gnome-session-fallback'. It can be installed from software center or synaptic package manager. If you are using UBUNTU 12.04, synaptic might not be installed with the default installation. So before installing ‘gnome-session-fallback’, ‘synaptic’ needs to be installed. (Before installing any software, make sure your system is up-to-date with command:
$ sudo apt-get update)
Search for synaptic in the search bar
All you need to do is connect to internet and click the install button:
After installing synaptic, search 'software center' or 'synaptic' for 'gnome-session-fallback', and install it.
Change your Desktop Environment
Now logout your current user and on the login screen, select the session as gnome-classic.
In the recent version of UBUNTU (e.g. 12.04), this option will be available by clicking the ubuntu sign near your username.
Select the gnome classic session, type in your password and you will get back your gnome classic interface.