What's the difference between package managers?

Probably the most popular package managers are apt-get, aptitude, synaptic, and Software Center. There are others (Linux Mint has its own, and there are some designed for KDE), but these are the ones you'll run into most often.

apt-get is a simple command-line tool. It's handy if you know the exact package name of what you want to install and don't want to spend time clicking through a GUI to get it.

aptitude is very similar to apt-get, and I've heard that it deals better with crazy dependency situations. Which one is really better is debatable.

synaptic is a low-level GUI. This is a good choice if you are a fairly advanced user but are not comfortable with command-line utilities.

Software Center is a very high-level, new-user-friendly GUI. Software is nicely categorized so that, if you're not exactly sure what app you want, you can find what you need quickly. The Software Center also stands out in that it is the only package manager in this list that allows you to purchase commercial applications.

dpkg is a lesser-used, low-level package manager standard for most Debian-based systems. In reality, apt-get, aptitude, synaptic, and the Ubuntu Software Center are all just front-ends to either dpkg or apt, which is in itself a front-end to dpkg.

In answer to your question, "which one is the recommended choice for normal day-to-day package management", I would say that Software Center is recommended for most uses. But as you gain more experience, you will find some of the features of lower-level package managers useful.

The Debian FAQ has a pretty good explanation of the different package managers. (dpkg, apt-get, aptitude, tasksel, synaptic)

As an addition to mac9416's excellent answer, Kubuntu offers the same command-line tools as Ubuntu, namely dpkg, apt-get, and aptitude. There are also two graphical package managers:

Adept is a straightforward GUI for apt-get, which lets you edit the source lists, browse packages by category or by name, see their status, and install/uninstall them.

KPackageKit is a simple GUI for PackageKit, which is a newer, cross-distribution package management system that uses apt-get behind the scenes. It lets you search for programs by various criteria, install and uninstall programs, make routine upgrades, and edit the source lists.

I personally prefer apt-get because it's a command-line program. And the syntax for installing packages is very simple:

sudo apt-get install packagename

I use apt-get on a day to day basis for installing and removing packages.

Synaptic is GUI-based and aptitude is text-based.

Basically, they only differ in how low-level they are.

dpkg: not even a proper package manager, apt uses it

apt-get: command-line, only if you know the package name

aptitude: text-based, but user friendly

synaptic: equivalent of aptitude in GUI

Software Center: for everyday users

BTW, these are all front-ends of apt (except dpkg), which is the only package manager on Ubuntu.

One other tool that has hardly received mention is tasksel. It's used for selecting specific tasks, mark them for installation, and then installing them (possibly using aptitude -- not sure).

Run tasksel --list-tasks to have a look at what tasks are available.

One key difference between aptitude and apt-get which has escaped notice above is that aptitude, unlike apt-get, will offer you different options in case there is a dependency clash. apt-get will simply fail. aptitude is much more flexible and versatile for that reason. Having never used a GUI package manager, I don't know how Synaptic and others handle such situations.