What is the difference between a corn and a callus? Is this a corn?

There was a callus-like thing in the center of the sole of my foot. At first, there was only one, but then two appeared. However, at some point, I started to feel something small and round. Moreover, recently, I began to experience pain, making walking uncomfortable. People around me are saying it might be a corn, but could this be a corn? If it is a corn, should I go to a dermatologist for treatment?