Radicular pain is often secondary to compression or inflammation of a spinal nerve. When the pain radiates down the back of the leg to the calf or foot, it would in lay terms be described as sciatica. This type of pain is often deep and steady, and can usually be reproduced with certain activities and positions, such as sitting or walking.
The pain usually follows the involved dermatome in the leg - the area of distribution of the leg covered by the specific nerve. When a nerve at the L4-5 or L5-S1 level is affected (bottom two levels), this dermatome is usually the sciatic nerve, which runs down the back of each leg to the foot.