There is a good, brief explanation of how the reserves work at the end of the linked article.
For the first time since the Gulf War, the Army is preparing for the involuntary activation of at least 5,600 soldiers who have long since hung up their uniforms, Pentagon and Army officials said Tuesday.
The Bush administration has mobilized thousands of reservists since the Sept. 11, 2001, terrorist attacks.
But this call-up, to be announced as early as Wednesday, involves troops from the Individual Ready Reserves, or IRR — servicemembers who have left active duty or active reserve service but still have time left on their obligation to serve (see "How the Army Reserve works" at end of story).
President Bush cleared the way for the Pentagon to use the IRR when he issued a Declaration of National Emergency after Sept. 11.
The declaration allows the government to order to active duty up to 1 million members of the Selected Reserve, Individual Ready Reserve, and Inactive National Guard without their consent, for a period not to exceed 24 consecutive months.