As with other swearwords and taboo words, or intensifiers, fuck is often not used in its original, literal meaning. Rather, it is an intensifier expressing nothing but the speaker's strong emotional involvement (often negatively, but not necessarily: e.g. "fucking good" is a rude way of saying "very good"). In the book Practical English Usage, the two meanings of the word are clearly illustrated by juxtaposing the sentences:

What are you doing fucking in my bed?
What are you fucking doing in my bed?
What the fuck are you doing in my bed?

The first sentence means "Why are you copulating in my bed?", while the second and third merely emphasizes the sentence "What are you doing in my bed?". The second usage is more common than the first, and the third is even more common then the second.

"Fuck you!" expresses anger, and thus seems to be more related to "I am so angry at you, I am going to rape you to punish you" than to "I would like to lovingly have sexual intercourse with you". It also may be related to "fuck off," which seems to be a reference to masturbation, where it might originally have been a vulgar way of saying "quit bugging me and go back to masturbating or whatever stupid stuff you usually do." It may also express indifference with respect to the well-being of another person or of other people in general, for example reacting to a request, or the imposing of rules.

Another use of the word fuck is as a replacement for the word God in profane statements. For example "Fuck knows" means something like "I don't know, and neither is anyone ever likely to know".


Linguistics

Verb

The word can be used as a verb transitively:
He fucked her.

Or intransitively:

They fucked all night.

Noun

As a noun:

He is a real fuck.
What the fuck?!

Interjection

The interjection fuck is frequently used to express shock, discontent and anger in general.

Fuck! A punctured tire!

Present participle

The present participle fucking (or fuckin' ) is commonly used to intensify a verb or noun. As described earlier, it is used more negatively than positively.

My fucking boss gave me tons of fucking work to do.
She is fucking beautiful.

In addition, the present participle is sometimes inserted in the middle of a word as an intensifier, a process known as expletive infixation. For example:

That was abso-fuckin-lutely cool!
Ex-fuckin-cuse me!
In-fuckin-credible

Portmanteau

Prepended to another word, the sound "f" is sometimes used to evoke the entire expletive, with an intensifying sense. (See portmanteau.)

That's fugly (fucking ugly).


Courtesy of Wikipedia.