Classic stealth is typically when you're using occlusion, camouflage, darkness, silence, disabling of detectors, and inanimate distractions (like throwing a rock to make someone look the other way) to hide your presence.
Social stealth is when you're using a crowd or disguise to hide your identity as a threat.
Basically, if it entails a person other than you and the observer you want to hide from (like an authorized person you're impersonating, or townsfolk you've riled up by throwing money to distract guards in an Assassin's Creed game), it's typically deemed "social."
Both can overlap and achieve the goal that an opponent doesn't know where the threat is. The trick with social stealth is they might be aware of your presence, but just don't identify you as a threat, because you pass for someone who's supposed to be there.