There are different types of markets and sellers out there. A monopoly has a single seller. He sells good and has no alternatives. Moreover, other firms are not allowed into the selling system either to continue the monopoly, at least not easily. These monopolies are created simply because of certain legal barriers put into place to provide stability.
It also has a unique product and there are strict restrictions on the exit and entry into the selling industry. In addition, to maintain privacy, the specialized information is confidential and cannot be leaked to other competitive firms or businesses, selling the same product.