Comcast is a cable provider and does charge a monthly fee. The amount varies depending on the number of channel in your package and the cable box you need.
Roku is box you buy that allows you to access internet streaming services and free "channels". Roku does not charge but the internet streaming services (Hulu, Netflix etc) do have a monthly charge. Roku allows you to access these services without using a computer.
Some cable providers have recently added access to some streaming services from the cable box.