The spiders scan the pages and add the keywords to their databases. You compete with thousands of websites daily for similar keywords. Apply keywords that match your content. Using misleading keywords is not recommended, and is risky. Register pages for all the individual search engines, and keep on top of changes.
Every search engine has a different method, and they change frequently.
Be careful not to use those quick one-time services without checking them out first. They do not believe they will find what they need on the first page. Poor descriptions are one of the reasons people are bypassing search engine links. One clear sentence or phrase describing who or what you do is valuable and necessary. The hope is that someone will click on your ad and come to your site.