A software bug is an error or fault in a computer program which makes it fail to work as it's supposed to. This could be as simple as failing to work at all, or as complicated as a subtly incorrect result. Nevertheless, we really want to eliminate software bugs, as they make programs less useful.
For an in-depth discussion about bugs, you should check out Software_bug.

Want to Help

One of the many reasons that Ubuntu is a great operating system is because it's developed by the community. You, me, and hundreds of other people join efforts in making it better each day. Bug triage is an excellent way to start helping out. You get to learn a lot about Ubuntu, its available packages, its infrastructure, and you get a feel for the development pulse.

Ubuntu receives an incredibly large number of bug reports every day through our BugTrackingSystem. Each one of these needs to be read, assessed, and sorted before being fixed in a process called Bug Triage. This is where we could use your assistance with HelpingWithBugs.

Responding to new bugs as they are filed.
Ensuring that new bugs have all the necessary information.
Assigning bugs to the proper package.
Confirming bug reports by trying to reproduce them.
Setting the Importance and status of bugs reports.
Searching for and marking duplicates in the bug tracking system.
Sending bugs to their upstream authors, when applicable.
Marking bugs that are feature requests as such.
Converting to a question a bug report that is a support issue rather than a genuine bug.

For more information check out this bug triage class that was given by experienced triagers Pedro Villavicencio and Iulian Udrea.

However, you don't need to do all of those things to help! Recreating a bug and setting the status to Confirmed or looking for a duplicate is enough. If you are experiencing a bug and want to submit it, please see Report a Bug.