Social Skills are a subset of skills that allow you to interact and communicate with other creatures. Social Skills are used to influence NPCs opinions, convince them to complete favors, and can be used to affect the morale of your opponents. Social Skills are necessary for negotiation, requesting favors, intimidating opponents, maintaining disguises, and making friends.
There are 5 Offensive Social Skills, each of which is opposed by one of the 5 Defensive Social Skills. Each Offensive skill can be used to adjust one of the 5 Dispositions or can be used in Social Encounters to Convince a character to complete a task or act in a particular way. Conversely, each of the Defensive skills is used to prevent Influence to the Dispositions and can be used partially used as a substitute for other Defensive Social Skills. Many of the Defensive skills also have minor active abilities.
Each skill is outlined in more detail on their individual pages, or you can visit the Social Encounters page to learn more about the Social system in general.