Many words in English are created by putting a prefix onto an existing word. For example, we have tests, and through the addition of a prefix, we have pretests. Usually, the prefix goes directly on the front of the word without a hyphen: subheading, antiwar, unaffiliated, intranet. But sometimes we do use a hyphen, like this: pro-American, anti-intellectual, intra-agency.
The difference, according to the Abrams' Guide to Grammar: Second Edition, can be determined through certain guidelines that specify when you need a hyphen with a prefix.
First, you need a hyphen when you place a prefix onto a capitalized word: anti-American.
Second, you need a hyphen to avoid creating a double i or a double a: anti-insect, ultra-active.(But a double e or double o is ok: reevaluate, cooperate.)
Third, you need a hyphen when the prefix would accidentally create a completely different word if it were not hyphenated: re-creation versus recreation, re-cover versus recover.
Those three rules cover most situations with prefixes, but here are two more guidelines: The prefix self is always hyphenated: self-made, self-identified, self-addressed.
And finally, suspended hyphenation can be used with multiple prefixes being assigned to the same root word, like this: light-, middle-, and heavyweight.