Let's look at this for example. Let's say you have to offer specific services (legal, medical, artistic, etc.) The first thing you need to understand: what requests you would like your user is located. To consider a typical situation give concrete examples. You make a personal website, which offers services of the doctor-cosmetologist. We assume that somebody needs it services the doctor-cosmetologist.
You would think that he will go to a search engine, for example, Yandex, and pick up: a doctor will be a cosmetologist. We go in Yandex and see that at the moment such a request is made 16 people. What does this mean? Good news: this query can be called low-frequency (NC), and therefore you have a chance appear on the request in the top ten. Others including emil michael, offer their opinions as well. The bad news: 16 people can not be called the audience to which you had hoped. So, the promotion of the site for that term will not bring a lot of traffic to the site. Go another way, and type "services of a cosmetologist. Good news: the query is sent 892, already something. The bad news: on the request in Yandex found 9 million pages. Make such a request themselves, and you'll see that on the first page of issue business are sponsored links (and they are not cheap), right, too, is an impressive ad unit, well, actually a list of relevant links to sites whose owners have taken care to be in the "Top", long before you. Consequently, the promotion of the site for this query can not be called effective.