Search engine marketing is very similar to search engine optimisation, well the only technical variation between the two terms in reality is the time periods in which they were the buzz term for the industry and the way and amount of money spent on each one. Search engine marketing tends to talk about large campaigns whereas SEO can apply to the smallest and grandest of marketing schemes.

When you say search engine marketing you should be aware this term incorporates things such as PPC campaigns which of course aren't part of SEO as PPC does not help you to rank any better in the engines unless you get natural links through the growth in traffic. SEM is a term which encompasses all of the SEO and the related forms of advertising to do with search engines. So basically search engine marketing is SEO and more. This is why many SEO companies wont run your AdSense of affiliate campaigns for you because this is the realm of search engine marketing. Some definitions place the meaning of SEM simply as a term for PPC and affiliate marketing whereas these are incorrect interpretations you should be made aware than SEM will sometimes be referred to in this manner.

Taking the latter definition, the narrow one, we can see that SEM is a useful but costly way to improve your websites traffic and as PPC is notorious for its abused payment system this just adds more negativity to this interpretation of search engine marketing, perhaps this is why SEO companies refer to themselves as SEO and not SEM in the current market or perhaps it is to do with the interpretation that SEM is expensive and only displays short term results.

The theory that SEM only gives short term results, and again we're going with the narrow definition here, is perhaps fairly well founded. The problem is of course due to the costs involved in PPC it is not often a cost effective long term method, while at 1st you may receive a boom in new customers that will not continue indefinitely. And when this happens you have to re-evaluate how much you should spend on SEM whereas the affiliate side of things is normally a safe bet as the affiliates only earn a share of the profits. So with affiliates you can't really loose. So whenever you speak of SEM make sure that you and the person you are talking to have the same definition of what SEM entails otherwise things may get confusing.