Sunnah refers to the Prophet's (SAW) life and actions, as a way of understanding the Quran. Sunnah isn't the same as the Quran's teachings. It's supposed to be a demonstration of how a Muslim should live, along with some clarifications. They complement each other.
From what I understand, Shias follow sunnah. The difference is that they disregard a few sunnah sources that Sunni's consider legit, because they don't agree with some of the companions who narrated them. A Shia would have more insight on the matter.