Crocheters can create any number of things like lace, fabric, or even jewelry with a crochet hook and some fiber (usually yarn or thread).
Hooks can be made from steel, aluminum, plastic, wood, bone, or bamboo.
Fiber can be made from cotton, acrylic, wool, or many other fibers.
Robot! Courtesy flickr user Erinn Simon [cc:by]
To crochet, you'll need some kind of fiber and a hook. Basic materials are available at any craft store and at some larger discount department stores.
Hooks come in a variety of sizes and materials. Here are some in plastic and aluminum.
Fiber also comes in a variety of materials...including cotton, wool, acrylic, and even plastic can be used to create something.
There are also specialized hooks and accessories that will help you with your crafting.
This is Tunisian crochet hook and a carrying case for a set of hooks.
You can also buy kits and patterns to get you started on something special. These are usually recommended for someone who has mastered the basic stitches.