How does the Navy differ from the Naval Reserves?
The same way all military branches differ from their reserve component. The Navy is full of full-time sailors who serve on active duty. Being in the Navy is what they do for a living. The Naval Reserves is a component of the Navy that is full of part-time sailors who serve one weekend a month and two weeks a year, and are available to go on active duty if they are needed to augment the strength of the Navy. They have civilian careers apart from their part-time service, and if the reserves are called up in a time of crisis (thousands of reservists have been called to active duty since Sept. 11th) their civilian employers are required to have jobs available for them when they return.
This is about 90% true. Technically, all officers entering the service nowadays (I think, anyway… at least this is true for the Academy and NROTC) are slated USNR rather than USN. They are still active duty, and their sole job in life is the Navy. Why is it like this? I think it’s due to legal issues and drawing down the service. If you’re Reserve (active duty), they can let you go without paying severance. If you’re USN, once you make Lieutenant Commander and beyond they have to compensate you for cutting you loose if you fail to promote. This is my understanding, anyway. You can augment over from USNR to USN once commissioned, but they charge you a year or two obligation for it.