Acrylic yarn is made from 100% acrylic fiber, which is just a synthetic fiber. It is perfectly safe, in terms of using it in projects or wearing garments from it. In fact, it may even be "safer" than other fibers because it carries a lower risk of allergic reaction than, say, wool. Acrylic yarn ranges from scratchy to soft, and comes in a wide variety of colors and price ranges. Most fiber artists (knitters and crocheters) do not like it as well as more natural fibers (like wool, bamboo, angora, etc.) because it is generally not as nice-looking as those. However, it is perfectly safe and readily available in many retail stores.
Acrylic is known as Acrylonistrille which we get from Acetylene or ethylene. It is also known as cheaper substitute of wool.