People who compile dictionaries are called lexicographers. While sometimes thought of as a branch of linguistics, the art of lexicography is properly considered a distinct field. Some are written by a single lexicographer, but many of the most respected and widely used examples today are the work of many individuals.
The lexicographer has many considerations to keep in mind when writing a dictionary. First of all, there are many different types of dictionaries with just as many intended uses. They may simply provide definitions, pronunciation, and basic origins — such as "Greek" or "Old French" — or they may provide more extensive derivations and histories of each term. Some, like the Oxford English Dictionary, provide textual examples of terms. These books may focus on specific subsets of a language, such as slang or legal terminology, or they may be used to give translations from one language to another. The first known dictionary, compiled in Latin during the first century BCE by Verrius Flaccus, listed only archaic and difficult terms.
Keeping the intended purpose of the book in mind, the lexicographer must choose which words to include, how much information to provide for each entry, and how to organize the data. Some aspects of organization seem fairly obvious, such as alphabetizing terms in an English dictionary or categorizing Chinese characters by radical and stroke count, a system known as lexicographic order. Alphabetized dictionaries did not appear in English until 1640, however, and earlier ones grouped words according to thematic similarity, such as listing all animals together.
There are also more subtle considerations regarding the organization of terms, such as how accented letters should be dealt with. A lexicographer must consider whether certain inflected terms, such as "children" in English, be listed on their own or included under the uninflected or lemma entry, "child" in this case. In some languages, all words with the same root are grouped together. In English, this would result in words like "important" and "report" appearing under the entry for "port" instead of under I and R respectively.
Some lexicographers have become household names, and revised editions of their work are still in use decades after their dictionaries first appeared. The most well-known of these are perhaps Noah Webster and Pierre Larousse.