Suppose I download a .deb package from a website and install it. (I assume that when I double click the .deb file, the package is installed through a GUI that interfaces with dpkg right?)
How can I uninstall it?
Manually installed packages appear in the Software Centre, along with all the others. Just search the software centre for your package and remove it there.
You may have to click on "Show N technical items"
Along with this, there are a few other methods:
Click the Apply button.
This will have the benefit of listing all of your manually installed packages:
You can either use
sudo apt-get remove packagename if you know the name of the package, or if you don't, search for it using
apt-cache search crazy-app and then remove it using apt get
You can also use
dpkg --remove packagename.
This will also let you know if there are any unneeded packages left on your system, which were possibly installed as dependencies of your .deb package. Use
sudo apt-get autoremove to get rid of them.
The command to facilitate that is:
sudo dpkg -r package_name
Open up Ubuntu Software Center (Applications -> Ubuntu Software Center) and search for the package you want to uninstall, and click on the Remove button:
Or from the command line:
sudo apt-get remove <package-name>
If your want to remove the package and all configuration files related to it:
sudo aptitude purge packagename
See this blogpost for more information.
The installed package will be visible in both Synaptics and software center. In the former one, it will appear under local packages and in the latter one under other.