SEO (search engine optimization) is the industry and practice that promotes websites in the search engines. SEF ("search engine friendly") is a loose term meaning something like "appropriate or not bad for SEO." For example, if you put your website content in Flash, it is not search engine friendly because the search engines cannot read flash.

There are various, but WordPress is the most popular.

It seems google currently favours wordpress so we have changed all our sites to wordpress and its really interesting to see the changes it made.