Arachnids are a class of animals. They include spiders, scorpions, and mites, which in turn include ticks. Arachnids belong to the phylum Arthropods. So they are something like the siblings of insects, millipedes, and crabs.
Arachnids have two limbs. They are easy to see in the web spiders and scorpions, and in the mites, they have grown together. Each arachnid has four pairs of legs, for a total of eight legs. Insects, on the other hand, only have six legs. Most arachnids hunt and kill their prey by injecting venom.
Most arachnids live on land. The water spider is an exception. For reproduction, many arachnids have some sort of penis, which they use to transfer their sperm into the female’s body. However, some also deposit their sperm packets somewhere so that the females can pick them up.