According to Brigham Young University's Writing Center, formal diction is language that is only used when addressing a highly educated audience, such as the readers of an academic journal. Informal diction is used when addressing a familiar audience, such as in personal emails or other documents with a conversational or entertaining tone. Informal diction often includes slang, while formal diction almost never does.
According to BYU, one example of the difference between formal and informal diction is the different synonyms for the word "learn;" while a formal equivalent to the word would be "edify," an informal equivalent would be the slang phrase "wise up." Choosing a style of diction is highly dependent on audience. Writing or speech intended for a highly specialized, highly literate audience – a medical lecture, for example – benefits from using more specialized language to explain more complex ideas. More personal communication – an email to a friend or family member, for instance – often uses informal language to show intimacy and friendliness. Diction is also informed by the purpose of the speech or writing. A financial grant request attempting to earn funding for a scientific project makes use of specialized language in order to show the seriousness of the researchers and their project. Conversely, writing or speech intended to reach a wide audience benefits from using simpler language (though it may not benefit from using extremely informal slang).