Correct, there's an audience for everything, even Mary Sues.
Or fetish sex that ranges from necrophilia, rape fantasies (read: bodice-ripper, only scarier), bizarre Star Wars/My Little Pony crossover, incest, Gorean cult... Ah, the things you find cruising the dark side of the 'Net.
What does the author want to achieve? which are her/his goals?
This is, IMO, the last thing one should aim for. Trying to educate someone via fiction (assuming that's what we're talking about -- non-fiction/essays are another matter entirely) usually ends up with the author looking helplessly pretentious or arrogant.
Win a literature prize? (Gee, how many of them did I have to buy and saw them rotting in the shelves <img src="/ubbthreads/images/graemlins/delight.gif" alt="" /> )
*smiles grimly* Same as above, usually. Anyone who sets out to try and win a literary prize is often quite deluded.
Which audience to target? Age, gender, genre preference
That's certainly relevant; fantasy targeted at adults is going to be quite different from one targeted at teens, and I don't just mean "mature" themes/sexual contents. (Though I, for one, avoid the "Young Adult" shelf like the plague, incidentally.)
Personally, when writing, I have no purpose beyond this: tell a story. No message, no theme, no goal in particular. I want to bring characters to life and make them real and human, and then I let them do what they want. If I put what I've written for public consumption, then obviously, I do that with a wish for improvement in mind.
Duel/dual -- a duel is a fight. "Dual" means two. *slappity slap*
Dessert/desert -- a dessert is something sweet; you do not trudge through a dessert unless you're an ant, and normally nobody puts sand, pyramids, or camels into a dessert.
Message/massage -- you use a message for communication. A massage is something else entirely.
Sentence/sentance -- "sentance" does not mean anything
Trust/thrust -- need I explain?
Thrusted/casted -- no, sorry, the past tense of "thrust" is
thrust, and the past tense of "cast" is also cast.