Title pretty much says it all. Wikipedia’s article National Guard (United States) - Wikipedia details the ever increasing federalization of the Guard. What does signing up for the Guard do that signing up with the Army Reserve doesn’t, or vice-versa?
Biggest difference, at least, IME: The Reserves are run by the Federal government, so all of the benefits that a person enlists for will be guaranteed regardless of where the Soldier lives or eve if the Soldier moves out of state during his/her enlistment contract. If a person joins the National Guard, many of the enlistment options cannot be guaranteed if the Soldier leaves the state of enlistment. The other big difference is that the Guard is called up more often than the Reserves. Fires, hurricanes, floods, generally the Guard gets called in first. The Reserves generally know weeks in advance of their duty rotations. So generally, those in the Guard must me a bit more flexible.
SSG Schwartz
Grr, corrected link: National Guard (United States) - Wikipedia