Bold textis a book designed to help the reader identify wildlife (plants or animals) or other objects of natural occurrence (e.g. minerals). It is generally designed to be brought into the 'field' or local area where such objects exist to help distinguish between similar objects. Field guides are often designed to help users distinguish animals and plants that may be similar in appearance but are not necessarily closely related.
It will typically include a description of the objects covered, together with paintings or photographs and an index. More serious and scientific field identification books, including those intended for students, will probably include identification keys to assist with identification, but the publicly-accessible field guide is more often a browsable picture guide organized by family, color, shape, location or other descriptors.
In 1934, Roger Tory Peterson, using his fine skill as an artist, changed the way modern field guides approached identification. Using color plates with paintings of similar species together - and marked with arrows showing the differences - people could use his bird guide in the field to compare species quickly to make identification easier. This technique, the "Peterson Identification System", was used in most of Peterson's Field Guides from animal tracks to seashells and has been widely adopted by other publishers and authors as well.
Also popular in the 1960s were the Golden Guides which expanded the range of subjects of what a field guide could address, including antique glass, wine, photography and hallucinogenic plants (often written by experts in their respective field - the latter was written by Schultes, a respected name in ethnobotany). This series was mostly edited by Herbert Zim for Golden Press.
Today, each field guide has its own range, focus and organization. Specialist publishers such as Croom Helm, along with organisations like the Audubon Society, the RSPB, the Field Studies Council, National Geographic, HarperCollins, and many others all produce quality field guides.
It is somewhat difficult to generalise about how field guides are intended to be used, because this varies from one guide to another, partly depending on how expert the targeted reader is expected to be.
For general public use, the main function of a field guide is to help the reader identify a bird, plant, rock, butterfly or other natural object down to at least to the popular naming level. To this end some field guides employ simple keys and other techniques: the reader is usually encouraged to scan illustrations looking for a match, and to compare similar-looking choices using information on their differences. Guides are often designed to first lead readers to the appropriate section of the book, where the choices are not so overwhelming in number.
Guides for students often introduce the concept of identification keys. Plant field guides such as Newcombs frequently have an abbreviated key that helps limit the search. Insect guides tend to limit identification to Order or Family levels rather than individual species, due to their diversity.