Google adwords is a pay-per-click (PPC) advertising. Meaning that you will have to pay per one click. In google adwords you will be able to create your own advertisement and target your niche by writing appropriate keywords in the ad. The ad will only be placed on the websites that relate to your keyword, for example if you want to sell youtube views and your keyword is "buy YouTube views" (you want to find customers who would like to 'buy youtube views'), than your ad will only be placed on the websites that have your keywords phrase of "buy youtube views" in their content. Additionally, your ad will be placed on the googles 1st, 2nd or 3rd+ pages of the search results. It will depend on how much you are going to pay-per-click. If it is high enough than your ad will be placed on the first googles search results page. You will see whether your ppc is high enough when you make your ad, google usually makes a note nearby your ad whether it is ok or if it need high ppc to get better results. For the keywords, you may either use google's keyword tracking tool which you can find here, or you can use alternative one which I use most of the time here. Additionally you can track the keywords of your main competitors. Again, for example you sell youtube views, than type youtube views or buy youtube views in google and select the first website that comes up. Copy the url of that website and paste into one of the keyword tracking tools, then you will see what keywords that website uses. Hopefully this was useful for you.
Good explaination. @OT: Adwords has inbuilt wizards to get started very easily. Give it a shot yourself and you will find it is not very tough to learn the basics. However it is very tough to get quality traffic at lowest price and requires efforts, skills and time.
Dear w4ebmaster Google is big bro and you can ask this question to google and he will lend you many tools for keyword option. https://adwords.google.com/select/KeywordToolExternal this might give you an idea.