Considering al Qaeda roughly means "the base" in Arabic, it is a broad organization of jihadists and mujahideen which pretty much anyone can fall under.
No, really, al Qaeda is considered a specific organization and there are other terrorist organizations at odds with them. Every now and then you read in the news of their bombings "accidentally" killing members of other insurgent groups in Iraq and then retribution bombings occuring. Some groups that the US/EU consider terrorist groups even make it a public point of distancing themselves from al Qaeda.
"Al Qaeda" is just another name that twists innoculous words into a title that serves supposedly justifiable political purposes. Of course it's not justifiable when you resort to violence to maim/murder innocent people to spread an idealogical message.