Compound adjectives

an adjective is used to modify a noun:

a pullover --> a red pullover
a holiday --> a long holiday

When another word (usually an adjective too) is used to modify the adjective, the two words together become a compound adjective:

a red pullover --> a dark-red pullover

Notice that often a hyphen (-) is used, but not always, especially in American English.

When numbers are used, they are singular and not plural:

a two-week holiday (vacances de deux semaines)

a four-wheel-drive a car

You can get more information about compound adjectives at wikipedia