The wet collodion process was a photographic process used to produce a negative. It was invented by F. Scott Archer (1813–1857) in 1848 and published in 1851. Prevalent from 1855 to about 1881, it gradually displaced both the daguerreotype and calotype processes (a process involving both a negative and a positive, introduced by William Henry Fox Talbot in 1841). Wet-collodion-on-glass negatives were valued because the transparency of the glass produced a high resolution of detail in both the highlights and shadows of the resultant prints (see image below). In addition, exposure times were shorter than those for the daguerreotype or calotype, ranging from a few seconds to a few minutes, depending on the amount of light available. Finished negatives were usually used to produce albumen prints, although salt prints were sometimes made during the 1850s and early 1860s.
Finette of Mabille, 1855, Nadar. Salt print from a glass collodion negative;  8 3/4 inches high x 6 1/8 inches wide (The J. Paul Getty Museum, 84.XM.436.496)
In the 19th century, the collodion used to coat glass plates was made from gun cotton, a commercially available product, which was ordinary cotton that had been soaked in nitric and sulfuric acid, and then dried. The photographer next dissolved the gun cotton in a mixture of alcohol and ether to which potassium iodide had been added. The resultant collodion was a syrupy mixture that could be easily poured onto clean glass plates as the first step in the production of negatives.
In the wet collodion process, collodion was poured from a beaker with one hand onto a perfectly cleaned glass plate, which was continuously and steadily tilted with the other hand to quickly distribute an even coating. The plate was of whatever size the finished print was to be, from a quarter plate measuring 4 by 5 inches to a mammoth plate measuring 18 by 21 inches. When the collodion had set but not dried (in a matter of seconds), the plate was sensitized by bathing it in a solution of silver nitrate, which combined with the potassium iodide in the collodion to produce light-sensitive silver iodide. After being placed in a holder, the plate was then placed in a camera for exposure while still wet—hence, the identification of the process as “wet.” After exposure, the plate was immediately developed in a solution of pyrogallic and acetic acids; a later refinement of the process used ferrous sulfate as a developer.
As some of these steps required darkness, photographers had to bring dark tents or wagons as well as chemicals and glass plates into the field with them. When enough detail became visible in the negative in the weak light of a darkroom, the negative was removed from the developer. It was then washed in water, fixed with a solution of sodium thiosulfate to remove excess undeveloped silver iodide, thoroughly washed to remove the sodium thiosulfate, and dried. With the addition of a protective coat of varnish, the negative was ready to be used to make prints.
First View of the Valley, about 1866, Carleton E. Watkins. Albumen silver print from a glass collodion negative; 16 3/16 inches high x 20 5/8 inches wide (The J. Paul Getty Museum, 85.XM.11.1)
That this complicated process was often used in remote places (see image above) by 19th-century photographers is a testimony to their diligence and dedication to their craft. This process enabled photographers to produce negatives in the field that could be brought back to a studio to be printed more than once.

Resources
Adapted fromLooking at Photographs: A Guide to Technical Terms by Gordon Baldwin. Los Angeles: J. Paul Getty Museum, 1991.
See the wet collodion process in the video The wet collodion process.