A compound is a word or word group that consists of two or more parts that work together as a unit to express a specific concept. Examples are double-check, cost-effective, around-the-clock, hand-to-hand, forward-thinking, eyeliner, and iced tea. They might also be formed from prefixes or suffixes, as in ex-president, supermicro, presorted, shirtless, or unforgivable.
Basically, compounds are written in one of three ways: solid (teapot), hyphenated (player-manager), or open (which ranges from phrases such as off and on or little by little to combinations like washing machine—have a field day finding more). Because of the variety in formation, the choice among the styles for a given compound represents one of the most vexing of all style issues writers—and lexicographers—encounter.
For some terms, it is often acceptable to choose freely among open, hyphenated, and solid alternatives, even though the term has been used in English for an extended period (for instance, lifestyle, life–style, or life style). Although the styling that ultimately takes hold for a compound may be determined by nothing more than editorial and writerly preference, there are patterns of new compounds as they become established in the English language. Compound nouns, for instance, are usually written as one word; compound verbs are generally written as two; compound adjectives are often written with a hyphen. But note that we added "usually," "generally," and "often"—we're hedging. (Be advised that we'll be using noncommittal terms throughout, and, essentially, that's the point of the following articles: there aren't fast rules to forming compounds, but there are patterns.)