Everyone who enlists in the United States Military, whether its for active duty (full time) or National Guard/Reserves (part time) incurs a MINIMUM eight-year service obligation. That's right -- when you sign on the dotted line, you commit yourself for eight years!
Whatever amount of time is not spent on active duty (which is full-time duty) or spent in the Drilling Guard/Reserves (those who drill one weekend per month, and two weeks per year, and are subject to be called to active duty), is spent in the IRR (Individual Ready Reserves).
In the IRR, individuals are not required to drill, nor do they draw any pay, but their names remain on a list and they can be recalled to active duty at any time, until their total eight year service obligation is complete. In fact, for the war in Iraq and Afghanistan, the Army has already recalled over 5,000 soldiers in the IRR back to active duty (so far, the Army is the only service who has recalled IRR).
For example, let's say you enlist in the Army on a two-year active duty contract. At the end of the two years, you separate from active duty. For the next six years, you are subject to recall to active duty at any time, if the Army feels they need you to help supplement active duty or reserve deployments.
Keeping the above in mind, the Army offers active duty (full-time) enlistment periods from two years to five years (only certain jobs are available for two and three year enlistees). The Navy will offer a two-year active duty enlistment, but they couple it with a two or four-year active (drilling) Navy Reserve commitment. The minimum active duty enlistment periods offered by the Air Force, Coast Guard and Marine Corps is four years.
Active (drilling) Reserve and National Guard enlistments are usually for a minimum of six years (if one wants the education benefits).
All of the services also participate in the Congressionally-mandated National Call to Service Program. Under this program, a member spends 15 months on active duty (full time), following basic training and job-school, followed by a minimum of 2 years in the active (drilling) Guard or Reserves, with the remainder of the total 8 year commitment in the IRR. However, all the services (except the Army) strictly limit the number of folks who can enlist under this program each year.