Single Keyword VS Keyword Phrases?

In general, you are better off targeting keyword phrases, and not single words.
For instance, if you are in Website Design, you would not use "Website" as a keyword.

WHY?
This is because if you went to Google or another search engine, and typed in "Website"... There would be too many results which are more relevant than yours.

Phrases are good, because you could use the words, "Website Design", "Website Development", "Website Designer" ETC...

This is better to do, than using one word keywords, because you have more chance in the search engines with phrases, than you do with single keywords.

Warning!
When you are working on your keywords, do not stuff them into the page, tags, or alt tags. Keep them evened out.

Do NOT stuff them in, keep everything in proportion and do your best to make sure you do a little research on your competition first.