SEO = Search Engine Optimization
SEM = Search Engine Marketing

SEO is the organic listing of the results you get when you search for a certain phrase. Some 80% of the people using the internet tend to prefer to click on these listings in stead of the sponsored links above or at the right hand page of the website.
SEO is pretty cheap, but of course there is a downside … it takes a lot of patience to get to the top!!

SEM on the other hand can be fast!! This is the sponsored or paid listing of the results you get when you search for a certain phrase. Tools like Google Adwords for example can be up and running in like 5 minutes, but they do tend to need constant attention and could turn out to be pretty expensive.
You could argue that the marketing research could take up quit some time, but it’ll still be faster than SEO, and a lot more expensive!