What is Al Qaeda?

Seriously. What is al Qaeda? Is it an organization over which bin Laden exercises operational control? Is it the group that al Zarqawi spawned in Iraq? Are they the same thing? Is it any group of angry young men that blows things up (or attempts to) in the service of radical Sunni goals?

Whatever they are, they are supposedly on the rise. Given though that a coherent definition is lacking, how do we even know what is on the rise? Is al Qaeda whatever the National Intelligence Estimate says it is? If not, who defines it? It's becoming so broad a term as to be nearly useless.