Suppose there is a pulley of mass m and two blocks are suspended on both sides of the pulley. Now what are the forces that the string applies on the pulley?

I can visualise the normal force acting towards the centre of the pulley. I wanted to know about the forces that are tangential to the pulley. Some sites state that those are tension due to the string and friction. I can understand how friction might play a role,but I am having hard time understanding how the string can apply the tension force on the pulley.