Any license comes typically with at least one rating, most of the time that being your "Single-Engine Rating," essentially, the kind of airplane you trained on. That permits you to fly airplanes with a single engine only.
Multi-Engine Rating: This rating is needed if you want to fly airplanes with more than just one engine. The primary difference between single- and multi-engine aircraft are the asymmetrical thrust conditions you encounter if one engine were to quit on you. This rating usually takes approximately 10 to 15 hours to obtain.
Variations of the ratings above, which I implied to be "land-based airplanes," are "sea-based airplanes," so you can actually get a "single-engine land," "single-engine sea," "multi-engine land," and "multi-engine sea" ratings. If you are so lucky have a plane that can do both -an amphibious airplane- you will need both ratings if you want to operate on both land and sea!
IFR (Instrument Flight Rating): This will permit you flying through clouds and instrument approaches, significantly increasing your reliability & safety. To obtain this rating typically takes 40 to 45 hours.
Type-Ratings: Whenever you fly a jet-powered airplane, or an airplane certified to operate with weights in excess of 12,500 lbs, you will need a specific type-rating. This rating is type-specific, but the expenses are often borne by the company that hires the pilot.
Flying Gliders (sailplanes) is extraordinarily beautiful! It is the perfect blend between sailing and flying. I'd be more than happy to steer you in the right direction, in case you are interested (http://www.ssa.org).
