Ok I just checked
Bulbapedia a while ago, and since that source is taken to be credible, I'll update on strengths and weaknesses.
Offensively, Fairy types are strong against Fighting, Dragon, and Dark types, and weak to Fire, Poison, and Steel types.
Defensively, Fairy types strong against Fighting, Bug, and Dark types, weak against Poison and Steel types, and
completely immune to Dragon types.
Some of these make sense to me, others don't.We already knew the first goal of Fairies was to cut the Dragons down to size, which explains the huge advantage in their favor. The advantage against Dark types makes sense, since in much mythology fairies are thought to be good and defend against the forces of evil. But on the flip side the advantage against Fighting type also makes sense, given fairies are also seen as tricksters, and melee attacks don't usually do much good against mythological fairies. As I and others have theorized, Fairies are in fact weak to Poison and Steel, given those two types' affinity towards the anti-natural.
What I don't get are the properties against Bug and Fire types. What would cause fairies to be strong defensively against Bugs but only normally effective against them offensively? Or what would cause them to be weak offensively against Fire but normally effective defensively? Can someone help me out here? Cuz this part seems kinda random to me.