Social media marketing is any sort of activity your business takes on social media that is unpaid or organic. This can be posting content on your channels, answering messages, liking or commenting on posts.
On the other hand, social media advertising is any form of social media activity that is paid, like boosting posts throughout a social network or running a targeted campaign at a specific audience (eg. location, household income, hobbies, etc.). You are paying to get more visibility or to get in front of users who may not be following your brand.