Two concrete and very common examples would be Facebook and Twitter. Both are social media and both are used extensively by marketers. Hence, when marketers use such media (as opposed to traditional ones like TV, radio, billboards, magazines etc) to target their consumers, that would be social media marketing.
Social media marketing refers to the process of gaining website traffic or attention through social media sites such as Linked in and Face book. There are also websites that specialize in promotion of small businesses.