Many new folks wonder: Why would I pick Ubuntu over Debian, or the other way 'round?

Ubuntu is a Debian-based distribution. The two are very much alike, but their goals are not exactly the same. To understand the differences, you'll want to know the goals of each. And you'll want to know which part of Debian is used as the basis for Ubuntu, and therefore which part of Debian is most similar to Ubuntu.

Debian releases include:
-Unstable (a.k.a. Sid)
-Testing (currently a version called "Squeeze")
-Stable (currently a version called "Lenny")

Ubuntu releases include:
-non-LTS releases (based on Debian Unstable)
-LTS (Long Term Support)* releases (based on Debian Testing)

1. Debian Unstable can change substantially every week. Why not? Sometimes it just breaks, and it's intended for people who don't mind that the software is not ready to be put in a "reliable easy to use" setup. Many users of Unstable are software developers.

2. Ubuntu non-LTS releases take the collection of Debian Unstable software, every six months (except when there's an LTS release). They take the stuff, try to fix as many bugs as possible, and make it "reliable & easy for beginners to use" BUT still based on very up-to-date software.

3. Debian Testing releases take the most-desired software from Debian Unstable software collection once every couple of years, and they make it work pretty reliably for people who are not computer experts. It is also not hard to use, but if you wouldn't know a GNOME from a KDE* from a LXDE, and you don't have any idea how to answer the question "do you want OpenOffice* or Abiword or Crossover Office?" then Ubuntu will probably be easier for you to use. A Debian Testing release comes out at the same time as the most recent testing release becomes a Debian Stable release. This happens "when it's ready"... once every few years.

4. Ubuntu Long Term Support releases come out every two years, and use the collection of programs/software from Debian Testing. These releases are generally easy for beginners to use, and very reliable. Desktop software for these versions of Ubuntu is supported for three years. (that means they get security updates, and there are folks who can help fix software problems that come up).

5. Debian Stable releases do not come with brand-new software, and they are about as reliable and predictable as operating systems get. These releases might support every piece of software you want to use (word processing, web browsing, web-server type stuff) but if you want a lot of new software, Debian Stable probably isn't for you. Each time a release becomes the "Debian Stable release" it is likely to be supported for for three or more years after that.

*feel free to look these terms up in google... "Ubuntu Long Term Support", "Linux Gnome", KDE, Openoffice, etc.