What is a lacquer?

In a general sense, lacquer is a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required.
The term lacquer originates from the Portuguese word for lac, a type of resin excreted from certain insects.  In modern usage, lac-based varnishes are referred to as shellac, while lacquer refers to other polymers dissolved in volatile organic compounds (VOCs), such as nitrocellulose, and later acrylic compounds dissolved in lacquer thinner, a mixture of several solvents typically containing butyl acetate and xylene or toluene.
While both lacquer and shellac are traditional finishes, lacquer is more durable than shellac.

Quick-drying solvent-based lacquers that contain nitrocellulose, a resin obtained from the nitration of cotton and other cellulostic materials, were developed in the early 1920s, and extensively used in the automobile industry for 30 years. 
These lacquers are also used on wooden products, furniture primarily, and on musical instruments and other objects. The nitrocellulose and other resins and plasticizers are dissolved in the solvent, and each coat of lacquer dissolves some of the previous coat. Lacquer grade of soluble nitrocellulose is closely related to the more highly nitrated form which is used to make explosives.
Lacquers using acrylic resin, a synthetic polymer, were developed in the 1950s. Acrylic resin is thermoplastic, obtained by the polymerization of derivatives of acrylic acid.  The use of lacquers in automobile finishes was discontinued when tougher, more durable, weather- and chemical-resistant two-component polyurethane coatings were developed.
Single pack lacquers cure by solvent evaporation only, and will generally remain soluble throughout their life span, and hence prone to damage from chemicals or moisture exposure. 
A more modern version is acid catalysed lacquer, a two-pack lacquer that incorporates a chemical catalyst to cure the coating to increase solvent and abrasion resistance.
In practice, a well applied lacquer coating should be indistinguishable from a polyurethane coating, but there is a vast difference in the performance characteristics between single-pack and two-pack products such as water resistance, solvent resistance and abrasion resistance. 

Industry standard for any commercial application would be to use a 2-pack product, either an acid-catalysed coating or a polyurethane.