Woodworker's will typically say "apply finish" but I started referring to it as a "sealer" just to reduce confusion. A sealer is typically a clear, film forming product that makes the piece easier to clean. It'll also stay brighter over the years and it keeps the dimensional stability more uniform. The old school clear products were shellac, varnish and laquer. Shellac will become sticky as it breaks down, varnish is not readily available and laquer is typically a spray product. Most of the time your current choices will be urethane or polyurethane in either a water borne or oil based product. The water based poly is a lot easier to use but it does not "enhance" or "pull out" the rich red and brown tones like an oil based product. Depending on your state and if they adopted the EPA VOC compliance some of the traditional oil based products are much harder to use because they run and sag profusely. In New York I all but gave up and found an oil/water hybrid finish that is easy to use and enhances the color almost as well as oil. I typically brush on two or three coats of finish but you can obviously buy a spray can and have easier results because the EPA restrictions are lower on spray cans (to my knowledge.) Other alternatives would be an oxidizing finish like tung oil, danish oil,linseed oil etc. These are typically hand appled with repetitive coats. Most of these finishes will still collect some dirt and show wear areas. I have used a Formsby tung oil that was impressive in the past. If you use an oil, dispose of the rags in an airtight container to prevent spontaneous combustion.