First of all, it's important to realize the difference between them and giant spiders. Giant spiders, while monstrous in their own right, aren't real monsters and don't have any real intelligence besides that of an animal. They can be anywhere from the size of a common rat to the size of a large dog, and their temperaments, feeding habits, web patterns, and toxicity all vary as much as that of normal spiders in the wilds of the human world.
Spider monsters, or 'spider people' for the politically correct, are true monsters, having heightened intelligence, strength, resilience, and the ability to take human form at will. Not particularly social creatures, they tend to have a fondness for reading and the arts. If you were to compare them to human high school students, they'd probably be those annoying Goth types, moaning about the pain of life while listening to awful grunge metal and wearing enough black to embarrass a shade.
In battle, the spider monster likes to ambush its prey, but if finding itself in an actual fight it will usually opt to immobilize its enemy with its webs before striking with lethal poisons. In combat, spider monsters spit nets of webbing to immobilize their prey before attacking with powerful venoms. A spider monster's webbing is as strong as steel cable, and its highly acidic venom, although usually non-lethal in the short-term, can paralyze and weaken even the mightiest of demons! Some monsters develop special abilities that allow them to spit poison too, or control swarms of normal spiders, or form webbing balls that explode into nets that wrap around enemies.
However, spider monsters are not aggressive fighters, and will only engage in battle if they have a significant advantage or if they have no choice. A spider monster that senses a threat to itself will always be careful to leave itself an escape route, if it doesn't flee outright. These cowardly traits, however, just spur these eightlegged monsters toward more extreme and more treacherous behavior in their efforts to avoid a fair fight at all costs. Spider monsters are quick to escape if they think they're outmatched, however, using their webs and many limbs to get past difficult obstacles and get out of reach.
Spider monsters are predatory monsters that live all throughout the world, and are generally well-versed when it comes to living among humans. They are masters of stealth, traps, and other deceitful tactics. Spider monsters eat all manner of creatures, including humans. However, the females of the species have gained a particularly nasty reputation, as they often use their human forms to seduce human men into their lairs before trapping and eating them. Because this tactic doesn't work nearly as well for the male monsters, females usually eat a lot more humans.
Finally, because they favor isolation, some of them take it to an extreme and become hermits, living all alone for decades. Overtime, they lose many of the skills of 'civilized' monsters and forget how to speak or take human form.
Eventually, they simply become wild beasts, indistinguishable from the giant spiders.
Such individuals are rare, as they go out of their way not to be noticed, but sometimes the unwary may stumble upon them, almost inevitably while wandering through ancient ruins where they don't belong. Anybody stupid enough to do such a thing should prepare to battle the creatures, as you're sure to be mistaken for a hapless adventurer looking for gold.