Let's examine the role each of these characteristics plays in explaining firm behavior.

One Firm: Because there is only one firm, this firm's demand curve is the demand for the entire market or industry, a typical downward sloping curve. This means that, in order to increase sales at any given level, the firm has to lower the price it is charging. We explore this in detail in the section on Marginal Revenue.
No Product Differentiation: Since there is only one firm anyway this isn't really important.
Restricted Entry: As we have learned, profits are the incentive for entry. The only way a firm can remain profitable and a monopoly is if there are very strong barriers to entry, or if the market is too small for a second firm.