Biological therapy is a type of treatment that works with your immune system.
It can help fight cancer (eg Rituximab for lymphomas) or help control side effects (how your body reacts to the drugs you are taking) from other cancer treatments like chemotherapy (eg G-CSF helps to increase white cell counts after chemotherapy).
While biological therapy may seem like chemotherapy as they are both given to fight cancer, they work in different ways. Biological therapy helps your immune system fight cancer. Chemotherapy attacks the cancer cells directly.
Biological therapies use the body's immune system, either directly or indirectly, to attack cancer cells. Click on the following link for a great summary of the types of biological therapies and how they are used: