A blanket term that refers to increasing the presence of your brand on the many and various social media sites, such as Twitter, Facebook, Pinterest, Google+, etc.
There is often a crossover between SMM and SEO, as the former is a great way to build links for SEO purposes. However, it is important to take a very careful, structured approach to SMM. It is fairly easy to waste a lot of time with it through lack of focus or measuring the wrong metrics (or none at all).
Contrary to popular belief, social shares themselves have precious little effect on SEO, although social media in general is having an increasing impact.