Dictionary of English - hyphen

Not all compound words are hyphenated, but the hyphen is needed to link adjectives that jointly qualify a noun or nouns: red-hot poker; Mrs Brown's twenty-odd nephews and nieces. Without the hyphen, the adjectives would apply separately to the nouns: a poker red in colour and hot, nephews and nieces twenty in number and odd in character.

Some compounds contain more than one hyphen: mother-in-law; up-to-date. There are no definite rules about when to hyphenate compound nouns. For example, you could write worldview, world view, or world-view. Generally, the more closely associated the words become, the more appropriate it is to merge them. A hyphen may be considered a middle way between separation and complete integration, but the modern tendency is to use fewer hyphens in compound words.

A hyphen is also used to mark a word break at the end of a line, to avoid a space or an ugly squashing-in.