Info on SEM

I really have no idea about SEO and the things that are connected with this thing. I have heard that it is all about making a site very much visible out there in the online world these days. The competition between sites like mine is totally elevated. Now, I also got informed that SEM or search engine marketing is one of the most important things being used for this SEO stuff. Now, I want to get to know better about SEM. I hope that someone from this online forum site could extend any form of help to me regarding this subject or issue.