The search engine marketing field is filled with terminology that is alien to most other marketing people. The discrepencies can lead to misrepresentation and confusion.
Search engine marketers or SEM's are professionals who are viewing all of the opportunities available on the search engines such as optimization, keyword buying, link building and even sponsorships or partnerships. Search engine optimizers or SEO's are focused on positioning your site on the search engines in the "organic" listings.
Choosing the right search engine marketer can be a complicated task. Knowing search engine terminology will help with your decision.
There are large companies that do search engine marketing and small one-person shops. The key is the level of attention that your site will get, and one person may be able to give you more individualized attention than a large company. Don't make your decision based on the size of the firm, but rather on the quality of their work.
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Other links relating to ethical search engine marketing:
Searchenginewatch.com article by Danny Sullivan