What group does spiders and ants fit in?

Spiders, and insects such as ants, are in the kind of group called a Phylum. The name of the phylum that includes spiders and insects (like ants) is the Arthropoda. The Arthropoda also include creatures like crabs, scorpions and other animals without bones, but with hard shells and jointed legs.

When one speaks of more than one phylum, you call them phyla. Other phyla include the phylum Mollusca, which is the phylum of slugs, snails, squids and so on. The phylum Echinodermata includes the starfish, sea urchins and sea cucumbers. Humans and other mammals, reptiles, birds, fish, etc are in the phlum Chordata.