Under the Class Arachnida, which means they're similar to spiders but they are not "true spiders". They reside in the Order of Opiliones, which are harvestmen.
Harvestman are also called "daddy longlegs" in certain regions. Harvestmen usually forage on dead insects, but are opportunistic hunters if the prey is easy to catch. They can be found under logs and rocks during the day, almost always in dark, damp, cool places. They do not have silk glands nor to they have vemon so they are completely harmless.
Interesting Tid Bits
This particular Opilione is known to secrete fluids when disturbed, but I personally have not experienced this. The quinones are used to deter predators from ingesting them, and there is a theory that the irritants are diet based rather than genetically synthesized.
I know there is a myth still around that the harvestmen is one of the most deadly spiders in the world, and it contains deadly venom, but their fangs are too small to puncture human skin. This of course is incorrect. Harvestmen do not have venom glands and their chelicerae are actually claws.
Keeping Your Harvestman Happy
These guys are pretty small, so a 2 1/2 gallon tank would work just fine depending on the number you keep. Since they normally live in damp, cool places keeping your tank on a lower shelf would be preferred. Keep the tank moist but I wouldn't fret about the humidity levels. I spray the tank at least every other day, but my tank has a glass top. The rule of thumb when checking to see if it's humid enough(without using a gauge) is this: If you can see water clinging to the sides or top of the tank you're fine. I use an Eco Earth and moss mixture for substrate and it seemed to help hold moisture.
What Do They Eat?
Nearly anything. I feed them a mixture of recently deceased crickets and fish flakes, but others have mentioned they will eat dog and cat food. They will run to get whatever you put in their tank almost immediately, and they also squabble over their spoils. They will also go after small wingless fruitflies if you want to keep those around.