We keep an eye on what searches are being used to find our website. The result is often fascinating and can be amusing. One of our top searches is 'website designing in surrey'. We have to admit this is not one we had targeted.
Trying to guess what people might use to find you is tougher than you might think. There's a lot of science, maths, statistics and quite frankly psychology involved. I think it is always worth asking your friends who are not involved in your trade what they would use if they were looking for your service. Not very empirical admittedly but can be quite informative.
Taking our case as an example, what would you use? The top search is 'Web design' with 7,480,000 searches worldwide, 14,800 for the UK. But we should keep in mind that every web designers website will be targeting that phrase.
The reality is that the keyword element of SEO, or search engine optimisation, is just one part of a far more complex subject about how to get people finding your website on the internet, enough for several books.