This removes the tomboy package, but it's configuration files and installed dependencies will remain in the system.
apt-get remove tomboy
This removes the package as well as its configuration files. But the dependencies will remain in the system. However, configuration files in the home directory won't get removed.
apt-get purge tomboy
This command is same as using --purge option with the above remove command.
This will remove the package as well as its dependencies, if those dependencies aren't used by other packages.
apt-get autoremove tomboy
If the package name is not specified with autoremove, it will remove all the unused dependencies in the system.