Probably the more common type of coating because it's the oldest. Many offset presses have aqueous coaters built right in. The coater is essentially a modified color unit in the press that applies a varnish rather than ink. Right behind the coater unit would be some sort of drying system, usually an infra-red lamp array that bakes the moisture out of each sheet as it passes through. Coating on the press is great; it can save time as coated sheets can be quickly flipped over and reprinted due to reduced dry-time, not to mention the improved final look.
Ultraviolet coating is quickly becoming popular due to the boom of toner based print engines. Because toner stays near the surface of printed materials, it's much more important to protect the printed piece if it has to survive tough conditions. Tough conditions like mail stream, outdoors or around moisture. UV coating is named so because it is cured instantly by a ultraviolet light. Chemically at it's core the coating is a wax but constituent solutions used for curing can release harmful vapors like ozone. For that reason UV coaters should be vented for safety. This method of coating is an extremely cost effective way to protect and enhance the experience.
Silk screen is great for spot coating. Those familiar with silk screen know how beautiful it can look. It can be rather labor intensive to achieve that quality. It also requires the preparation of a custom screen for each spot pattern.
Spot and Flood
Flooding is when the entire sheet, edge to edge, is covered with a thin layer of liquid coating. Flood coating is more common because it's easier to do and protects the entire sheet. Spot coating looks beautiful. It's a targeted coating that usually lines up with the printed image. Think of a photo of a glass of beer, where the droplets of water glisten on the paper from a spot coating. Dang, I'm thirsty. Selling the sizzle dramatically improves the effect of printed materials and advertisements. It can be expensive but the effort pays off in spades. MGI's Jetvarnish 3D is a dramatic game changer in spot coating. The method of applying the coating is inkjet! This reduces costs by eliminating tons of time and waste.
Wow! Now we're getting into some fun stuff. 3D coating brings actual tangible texture to flat sheets. Literally printing texture with clear coatings adds depth and engages the reader. A basketball that, when you run your fingers across, feels like a real basketball. Rough textures like dirt or wood, as well as smooth glossy things like a dolphins skin can be right next to each other on the same image. This can also be a replacement for the traditional method of embossing. Embossing brings textures to romance novel covers at the grocery store but embossment plates are incredibly expensive and are only even possible on extremely long runs.
Coating vs Laminating
This really should be an opinion article; there are a thousand factors that should influence this decision and really each scenario is unique. There are some general guidelines that can help. Laminating is much better at protecting, applications like book covers and restaurant menus that undergo absolute torture are great examples. Coating, however, is less expensive, it's faster and it still looks great.