Keywords, how many should I subscribe to?
Google allows subscribing to keywords based on broad match, phrase match and exact match and several points in between. With a broad match, an advertiser only needs a small set of keywords to have their Ad shown often. For example. If you sell "Tennis Shoes" and select broad match your Ad will show whenever the word Tennis or the word Shoes is used as a search term. When a visitor search on "tennis court construction" they will see the Ad for tennis shoes. They may be a prospect but it is unlikely they are a buyer. If the advertiser chooses exactly match the Ad will only show when both words are present. If someone searches on "Nike shoos" the Ad will not show. An exact match will generally result in more qualified prospects. A major sporting goods store could easily create a Google campaign with thousands of keyword phrases, one for each item stocked in the store. This is a potential management nightmare, but it may be worth it if the Ad budget is large enough. As a compromise, Google has provisioned the use of - + associated with each keyword. The search phrase "tennis +shoes" will result is the Ad showing for any search term that includes the word "shoes" so the Ad would not show on a search for "tennis court construction" but it would show on "dancing shoes" -dancing +shoes would be displayed for all search phrases containing shoes and not dancing.
The preceding paragraph is a simplistic representation of the process. Someone searching with the term "hiking boots" would also see the tennis shoe ad. Google considers synonyms when serving ads. The word shoe has 18 synonyms.
My advice to clients is to generate hundreds (possibly thousands) of exact match keyword phrases for their Google AdWords campaigns. About 10 years ago the website cars.com was managing 500,000 keywords in their Google Adwords campaign.
The average small business, small-time advertiser identifies a dozen key phrases then sets up the broad match as a matter of expediency. Resulting in a higher cost per conversion than the better organized Google advertiser.
My advice, go for the long tale, exact match phrases. If you sell televisions don't buy the phrase television, buy several phrases for each model television you stock. If someone searches for Sony Model MXer45 they are further along in the buying process than someone searching for the phrase smart tv.
Google keeps track of all search terms used for the 3.5 billion searches that take place each day and they make it available for phrase selection and planning.
Organic search ranking for a website is slightly influenced by the presence of the same search phrase employed by the Google search visitor but there are also 199 other factors considered by Google when determining search result ranking. Google robots use a proprietary algorithm to perform symantic analysis of website content. Rather than to try to second guess the robots it's better to focus on creating quality content for humans with the understanding that the robots are trying to simulate human understanding.
This is a credible short video on the subject by Rand Fishkin - a well-known authority on the subject of search engine robot behavior.