A buzzword is a term or phrase that becomes very popular in some specific context. Buzzwords are typically either newly coined words -- often acronyms or portmanteaus -- or existing terms used in a different way than they were in the past.
Sometimes a buzzword is a new term for a popular new technology or concept. The word wiki was created, for example, to refer to server programs that allow collaborative online content creation and editing. Although the word is often considered a Web 2.0 buzzword (as is Web 2.0 itself), it's a useful term. Blog is a similar example.
Other buzzwords are more disposable, popular for a time and then becoming outdated and/or replaced. Paradigm shift, for example, became very popular to describe a substantial change in approach; that term was largely displaced by sea change, which has in turn been displaced by transformation.
See also: Buzzword bingo, a passtime designed to mitigate the boredom of attending corporate meetings where jargon and buzzwords abound. Apparently, this game is still being played.
How can you improve your buzzword game? Set it to music: