Spider is a creature type.
Spiders are mostly Gaint Spiders. Almost all of them are able to block Flying (Reach) (exception: Giant Trap Door Spider) and some of them, like Woolly Spider, get a bonus when they block Flying Creatures. Their Toughness is higher than (or equal to) their power, but rarely lower. The Strength of spiders isn't power/toughness, like with Wurms, but rather Reach and high(er) Toughness => durability.