Modifiers are words, phrases, or clauses that provide description in sentences. Modifiers allow writers to take the picture that they have in their heads and transfer it accurately to the heads of their readers. Essentially, modifiers breathe life into sentences. In grammar, a modifier is an optional element in phrase structure or clause structure; the removal of the modifier typically doesn’t affect the grammaticality of the sentence.

Examples :

• [Put it gently in the drawer]. (adverb in verb phrase)
• She set it down [very gently]. (adverb in adverb phrase)
• He was [very gentle]. (adverb in adjective phrase)
• [Even more] people were there. (adverb in determiner phrase)
• It ran [right up the tree]. (adverb in prepositional phrase)
• It was [a nice house]. (adjective in noun phrase)
• His desk was in [the faculty office]. (noun in noun phrase)
• [The swiftly flowing waters] carried it away. (verb phrase in noun phrase)
• I saw [the man whom we met yesterday]. (clause in noun phrase)
• She’s [the woman with the hat]. (preposition phrase in noun phrase)
• It’s not [that important]. (determiner in adjective phrase)
• [A few more] workers are needed. (determiner in determiner phrase)
• We’ve already [gone twelve miles]. (noun phrase in verb phrase)
• She’s [two inches taller than I]. (noun phrase in verb adjective phrase)

A premodifier is a modifier placed before the head (the modified component). A postmodifier is a modifier placed after the head, for example:

• land mines (pre-modifier)
• mines in wartime (post-modifier)
• time immemorial (post-modifier)

http://www.chompchomp.com/terms/modifier.htm

http://en.wikipedia.org/wiki/Grammatical_modifier