Well the short version answer for that question would be that social media marketing is the use of social communities such as Digg, Stumbleupon, Propeller etc. to submit content to and receive links and traffic to your website in return.
Using these social media sites can bring you in large amounts of traffic but it’s not automatic. You don’t just throw up any old piece of content and expect to get placed on the front page of Digg or get hundreds of favourable comments on Stumbleupon. You must submit GREAT quality content.
Just think about when you are looking through these sites. Do you give just any old site the thumbs up on Stumbleupon? Heck no! Do you “Digg” an article because you feel obligated? Nope again! That’s the beauty of social media sites… we choose what we WANT to see and we reward the sites we LIKE. If we don’t like it… we give it a thumbs down, bury it, report it as spam etc.
So create great quality content!
Also, don’t just submit content all the time… actually use these sites the way they were intended to be used. Check out sites on Stumbleupon and give them a thumbs up or thumbs down. Go read articles on Propeller and vote for them or sink them. If all you do is submit articles all the time and don’t actually social bookmark, digg or comment on other member’s submissions, you will be looked at as a spammer and most likely kicked out.
So what do you gain from all this time you spend in these communities?
One of the most important things is that you will get high quality links coming to your site. Google loves these sites because of all the great content and it is possible that they could index your submissions in a matter of hours. If you have done your keyword research properly, you can get lots of traffic from a high ranking in Google.
You can also get a lot of traffic just from these social media communities alone. They get a huge amount of traffic reading through their content each and every day and if your article looks interesting, they will come to your site.
So if you haven’t already… go join these social media sites… spend some time in each one commenting, voting and making friends… then submit some great quality content and watch the traffic roll in 
