The Arachnida include the terrestrial chelicerates that everyone is familiar with, and that nearly everyone would rather not be too familiar with: spiders (Araneae), ticks and mites (Acari), and scorpions (Scorpiones). Arachnids also include a number of less familiar taxa: Opiliones (harvestmen or daddy-longlegs); Thelyphonida (whip-scorpions); Pseudoscorpiones (false scorpions); and many others. Most are predators, and some are venomous. All are terrestrial, except for some mites and spiders that have become secondarily aquatic.
For more information on arachnids on the WWW, try the Arachnida pages at the Tree of Life, or visit the very comprehensive arachnology home page at Antwerp University in Belgium and this fine Arachnida Sources Listing.
Peruse the excellent Scorpion du jour pages for information on scorpions, or see some spider pictures on the arachnid mailing list homepage.
Shultz, J.W. 1990. Evolutionary morphology and phylogeny of the Arachnida. Cladistics 6: 1-38.