Newbie Confusion about Keywords and How They're used.....

I am a newbie who is still a bit confused about keywords and how they're used.

I understand that keywords are what people enter into search engines to search for a particular area of interest, I understand how you research a keyword to find out how many hits it gets per month, but I'm a bit confused about how a marketer particularly uses a keyword for a site.

1) This may sound like a foolish question, but is there a specific tag used in HTML for keywords so that search engines can pick up on them??

2) Also, I've read how people write articles/blogs with keywords in them...what do you mean by that? E.g. if someone had a keyword "dog training", do they then write a blog with the words, "dog training" in them that will also subsequently be in the articles they duplicate and add to the different directories?

3) Now, because they'd be using these keywords as their "niche" so to speak, do they have to subsequently insert these keywords in EVERY blog they put up on their site? Won't that make the blog a bit stale if you're writing blogs for the purpose of inserting your chosen keyword?

4) What if 5/6 or whatever months down the line you decide these keywords aren't really searched too much or are too competitive, are you then screwed because all the other blogs that you have written have had these keywords and you can't go back and change them?

5) Can a site subsequently have more than one keyword?

6) What is it EXACTLY that guarantees your site getting listed high in the search engines with your chosen keywords? Is it the blog that you've written that contains the keywords that's posted on your site and on directories and social bookmarking sites as articles? Or is it that the backlinks that you're using to link back to your site actually have your keyword as the hyperlink back to your site. For example, if "dog training" is your keyword, you turn it into a hyperlink that links back to your site.

7) I heard it loud and clear that you can insert duplicate copies of your blog to EZA, Articlesbase, and other directories without any problem just as long as you are the author. How do you guys deal with squidoo, hubpages, etc. Do you also post duplicates on these sites as well or do you prefer to abridge what you've written. (Hey, if my question about squidoo, hubpages, etc. was already answered, forgive me, this was a LONG post. )

Sorry if my questions seems confusing, that is because I'm confused...

Thanks,

Ike