When looking at "keywords", you may wish to consider that one little letter at the end, the letter 's'. Getting on the first page is not about 'keyword' but 'keywords'!
Take for example, 'marketing', that is too vast a subject to get quality visitors to your site. A person using just the one word 'marketing' in their search, may have no idea what they are actually looking for or are just wandering what the term means - using the internet as a dictionary.
If you have a site on marketing, in order to stand out (B unique), your content must address some specific aspect of marketing that other sites do not address and also fill a need for a large number of people that are interesting in marketing.
Thus, your content will have a lot of words on your unique subject - a few of those words will be "Key" to people finding your information. Thus, the term - "keywords" (plural).
Long tail keywords are defined as keyword phrases that consist of between two and five words, usually used when searching for a rather specific item. Therefore, long-tail keywords have less competition and when your site have the correct keywords within its content and that content is unique - you will rank on or close to the first page of the search engine.
Plus, when the person that reach your site is actually looking for the unique content they find on your site, they will interact more with your site. The person doing a general search, say for 'marketing', may not find anything they think is useful on your site. They wont stay long which is bad for your page rank and if your site is set to produce income, they wont be there long enough to buy anything.
The trick is to make sure your content (the 'words' on your site) are 'key' to what you want people to search for in the first place.
Also see: Keywords for making money