As we age, our ability to focus on objects close to us deteriorates. This is a condition known as presbyopia. As we approach our 40's, most of use become short sighted. It's a normal part of aging. Anyway, if you are getting closer to that age when considering surgery, the clinic will probably offer to make one eye perfect for distance and the other eye less than perfect for distance.

This differential vision is called monovision. Your dominant eye is corrected to focus for distance and the non-dominant eye for closer objects. What this means is that your brain will generally switch between eyes depending on what you are looking at. This is common anyway, for people without perfect vision, with one eye generally being dominant.

Most people adjust to this system fairly quickly, and hence won't have to wear reading glasses as they age, since one eye will still be able to see things that are close up. Personally, since I'd worn glasses or contacts most of my life, I decided not to do monovision. I wanted good distance vision, and I won't mind wearing glasses for short periods of time if I need to read something close.

I have a relatively inactive blog about my own laser surgery if you are interested.

For more details, see the Wikipedia entry on ocular dominance, which is the principle by which monovision operates.