What is Lacquer?:
Because of the similarity of the names, lacquer is often confused with shellac. The confusion often comes from the fact that shellac comes from the lac beetle, but lacquer does not. Instead, lacquer is derived from the resin of a specific tree commonly referred to as a varnish tree. This resin is harvested from the tree's sap, and is then refined and mixed with lacquer thinner to create the common woodworking finish. Lacquer is commonly used with some paints to give a strong, durable paint finish.
Most lacquers used in woodworking today contain another resin called nitrocellulose, which (along with some other ingredients) allows one thin coat of lacquer to dissolve somewhat with a previous coat. After numerous coats of lacquer, the finish is very hard, yet somewhat flexible. The main drawback to nitrocellulose lacquer finishes is a susceptibility to ultra-violet light.
The introduction of nitrocellulose lacquers allowed automobile manufacturers in the 1930s to begin offering cars in colors other than the traditional black.
Lacquer is available for application with either a brush or as a spray. However, it is not a good idea to try and brush on a spray-on lacquer application, as the spray versions are much quicker drying. Brush-on lacquer will dry quickly, but is formulated to give you at least a little bit of time to apply and even out the finish. Use a bristle brush, preferably of high-quality natural bristles, to apply the lacquer. Be certain to work quickly adding a thin coat, but don't over-brush your work. You can add additional coats later to even out the finish as needed.
If you choose to spray on your lacquer, be certain to work in a very well-ventilated environment. The solvents in lacquer have a particularly strong odor, and are quite flammable.
Lacquer vs. Polyurethane:
Lacquer finishes are easier to apply than polyurethanes, as they can be a little bit more forgiving, particularly when brushing the lacquer. They dry much faster and tend to show far fewer brush strokes. While lacquer finishes aren't quite as durable as polyurethanes, they are much easier to repair if any damage occurs. As an added bonus, lacquer can be used to protect metals as well.
NOTE: Polyurethane and lacquer do not play well together. As such, you can't apply polyurethane over a lacquer finish or vice versa.