A "Bug" in Bugzilla refers to an issue entered into the database which has an associated number, assignments, comments, etc. Many also refer to a "Ticket" or "Issue"; in this context, they are synonymous.
Each Bugzilla Bug is assigned a number that uniquely identifies that Bug. The Bug associated with a Bug Number can be pulled up via a query, or easily from the very front page by typing the number in the "Find" box.
A Bug has stages through which it must pass before becoming a "closed bug", including acceptance, resolution, and verification. The "Bug Life Cycle" is moderately flexible according to the needs of the organization using it, though.
A Product is a broad category of types of bugs. In general, there are several Components to a Product. A Product also defines a default Group (used for Bug Security) for all bugs entered into components beneath it.
Example 1. A Sample Product
A company sells a software product called "X". They also maintain some older software called "Y", and have a secret project "Z". An effective use of Products might be to create Products "X", "Y", and "Z", each with Components "User Interface", "Database", and "Business Logic". They might also change group permissions so that only those people who are members of Group "Z" can see components and bugs under Product "Z".
"Q/A" is short for "Quality Assurance". In most large software development organizations, there is a team devoted to ensuring the product meets minimum standards before shipping. This team will also generally want to track the progress of bugs over their life cycle, thus the need for the "Q/A Contact" field in a Bug.
See: Infinite Loop
This is the cryptic response sent by Bugzilla when a query returned no results. It is just a goofy way of saying "Zero Bugs Found".