What is a paper cut?

From the papercut website:

The One Hundred Paper Cuts project exists to work on the little annoyances in Ubuntu. These bugs are normally considered too low priority for the developers of the apps in question, who already have more than enough to work on, and so the One Hundred Paper Cuts project comes along and picks them up. In each release cycle, the project aims to fix 100 of these little bugs and in doing so, give Ubuntu a layer of polish that is not typically found in other Linux distros.

As for the definition of a paper cut:

Put briefly, a paper cut is a trivially fixable usability bug that the average user would encounter in default installation of Ubuntu Desktop Edition.

If you prefer a more detailed, itemized definition, a paper cut is:

A bug, or an unintended problem occurring within an existing piece of software,

the presence of which makes a computer more difficult or less pleasant to use,

that is easy to fix,

that the average user would encounter...

in a default installation of Latest release of Ubuntu or Kubuntu, Desktop Edition.

If a potential paper cut fails to meet any of the criteria above, it is not a paper cut.

All the information on how to get involved is on the site too. See One Hundred Papercuts - Ubuntu Wiki for more information.

The definition of a paper cut

Put briefly, a paper cut is a trivially fixable usability bug that the average user would encounter in a default application included on the Ubuntu desktop. If you prefer a more detailed, itemized definition, a paper cut is:

A bug, or an unintended problem occurring within an existing piece of software,
the presence of which makes a computer more difficult or less pleasant to use,
that is easy to fix,
that the average user would encounter,
in a default application of the current Ubuntu, LTS or development release.

If a potential paper cut fails to meet '''any''' of the criteria above, it is '''not''' a paper cut.

How can I tell if a bug is easy to fix?

A bug is easy to fix if it can be fixed by one person in one day. In practice, one or more people might work together over the course of a week to fix a paper cut, but if one competent developer cannot fix the bug in a single day, the bug cannot be considered a valid paper cut.

Many complex bugs become trivially fixable right before they are fixed. If a bug appears too complex to be considered a paper cut at first, it may turn out to be trivially fixable if it has a working patch that could be cleaned up and merged by one person in one day.

It often takes a good deal of technical experience to know whether or not a bug is trivially fixable. If in doubt, send a message to the Paper Cut Ninja mailing list, or ask in #ubuntu-desktop IRC channel on Freenode, where someone will be able to point you in the right direction.

Who is the average user?

If you are reading this answer, or reporting a paper cut on Launchpad, chances are that you are not the average user. To understand who the average user is, check out the following articles:

Wikipedia article on Personae
What is an Average Computer User: Classic Linux User Thinking…
Five crucial things the Linux community doesn't understand about the average computer user - Part 1 and Part 2

What is not a paper cut?

A new feature is not a paper cut, a paper cut is a problem with an existing piece of functionality, not the addition of a new one.
The addition or removal of a package is not a paper cut. "Replace F-Spot with Solang" is not a paper cut, neither is "Install simple-ccsm by default."
A bug that the average user encounters once or never is not a paper cut. The more times a day the average user experiences the problem, the more likely it is that it's a paper cut.
A paper cut is not merely a really annoying bug. Just because a bug is really bothersome, and has gone unfixed for years, doesn't make it a paper cut — it's probably like this because it's difficult to resolve and no one has the courage to tackle it.
Localisation issues are not paper cuts. For a bug to be a paper cut, it must affect the majority of average users and localisation issues will only affect users in that locale.
- Issues affecting any English text, such as badly worded app descriptions in the Software Centre, can be considered valid paper cuts since the English text string are used as the baseline for all localisations. If the English is wrong, then the translation will be wrong.

Caveats

While a paper cut does have a definition, it's not too strict. If an issue lies on the border of being a paper cut and not being one, then report it anyway and the Paper Cut Ninjas will take it from there. If it satisfies one of the criteria for not being a paper cut yet you think it should be considered, then report it anyway and the Paper Cut Ninjas will decide what to do with it.

The paper cut team has a very open mind. If something looks and sounds like a paper cut, it probably is.