Use of abbreviation varies widely depending on the formality of writing employed for a given publication or a piece of content. Generally, the more formal the content, the less likely it is that abbreviation will be used, except in multiple references to terms commonly abbreviated or in tabular matter and other graphic elements.
In formal writing, journalistic contexts, and some informal content as well, terms are spelled out on first reference, followed by abbreviation in parentheses, as in “The Global Positioning System (GPS) uses satellite signals to fix the location of a radio receiver on or above the earth’s surface.” Thereafter, the abbreviation is used exclusively.
However, this tradition applies to single pieces of content, so that—unless, for example, an entire publication is devoted to articles about GPS technology—two articles in a publication that mention it will independently introduce the full spelled-out version of an abbreviation on first reference. Note, too, that specialized publications will likely abbreviate all references to widely used terms in that specialty.
Abbreviations consisting entirely of uppercase letters (including NY, US, FBI, and NASA) or that end with an uppercase letter (as in PhD) are not followed by a period; some publications retain periods in these types of abbreviations (at least two-letter ones), but that style is in decline. Abbreviations that end with a lowercase letter (a.m., Dr., i.e., etc.) are generally followed by a period.
Acronyms (abbreviations of phrases using initial letters of each word to form new word, such as AIDS) are almost invariably styled in all capital letters, though some, such as laser and scuba, have lost their uppercase form, and Nasdaq is treated as a proper noun. Initialisms (abbreviations of phrases using initial letters of each word, each of which is pronounced, such as FBI) are also generally capitalized. When using an article before an abbreviation, choose a or an depending on the first sound, not the first letter, of the abbreviation: “an NBA [en-bee-ay] team” but “a NASA [nasa] program.”
Avoid ampersands except in proper names (“Johnson & Johnson”) and in widely known abbreviations (“R&D,” for “research and development”).