The term "shikki" (漆器) traditionally refers to japanned objects: wooden objects or paper, lacquered with a type of sap called urushi (漆, from which the Chinese character in the name is derived).
Nowadays, the term is used to address a wide range of goods made in wood or metal, lacquered using either urushi or other vegetable resins.
The word nurimono (塗り物 or in hiragana, ぬりもの - lit. "coated object") is also used.

Overview
Lacquer coating has the simple purpose of making objects more resistant to environmental factors, such as temperature and humidity, and more suitable for daily use and washing. The high humidity found in several Asian countries made the technique particularly appreciated and used in China, from where it was imported to Japan.
Thanks to the freedom provided by lacquer when crafting and decorating objects from wood, the technique developed in Asia to create items of extraordinary beauty that may nonetheless be used on a daily basis, including plates, cups, boxes and small objects such as the inrō (印籠, the Japanese hanging case once used by Japanese men to carry small objects when wearing a kimono).
In general, Japanese lacquer finishing is not dissimilar from other techniques that are widespread in Asia, the only differences being in style and materials used for the object. Nonetheless, there are some specific techniques that are believed to be native to Japan, such as chinkin (沈金, see below).
Traditionally speaking, there is an astonishingly large number of techniques used to make shikki goods, including negoronuri and chinkin. The following list only includes a few common techniques; please visit this page (in Japanese) to see examples for each of the 155 techniques listed.
• aogaizaiku (青貝細工), when seashell is applied to the object before finishing it.
• negoronuri (根来塗), where a layer of red color is applied over the black layer, and then scratches are made to partially reveal the underlying black layer.
• chinkin (沈金), featured in the video below, in which gold powder is shoved into lines previously carved into the object.
• aizukinji (会津金地) and keshikinji (消金地), where gold powder is sprinkled over the object shortly before the first layer of lacquer dries, followed by a second layer of lacquer.

Aogaizaiku

Negoronuri

Aizukinji
Origin
Basic lacquering techniques were introduced from China before 7000 BC. The oldest artifacts were unearthed in the northern island of Hokkaido.

Authenticity
Traditionally, the term shikki is used for items made in wood, paper, bamboo and metal, lacquered using urushi lacquer. However, as the technique has been adopted for thousands of years, it is hard to declare or deny authenticity. Many manufacturers and retailers use the term to indicate objects that do look Japanese but are made in other countries, using plastic and lacquered using synthetic resins.
To us at KiGinKin, as well as to many Japanese people, the term ought to refer to objects made in natural materials and lacquered with vegetable resins, either urushi or the more modern cashew coating.