The demand curve of a perfectly competitive firm is perfectly elastic parallel to the x-axis and the demand curve of a monopolist is downward sloping, steeper, and less elastic. The main reason for this difference is because a perfectly competitive firm is a price taker firm and monopoly is a price maker firm. Because a monopoly has no substitute for its product/service it can charge a price that is favorable to the firm. Yet, the monopoly can't abuse market price as it has to choose a price-quantity combination that is available on the downward sloping demand curve. It can charge a high price but have to forgo a higher sales level. Remember that a perfectly competitive firm is selling a homogeneous product with many close substitutes available in the market. A competitive firm can sell as much quantity as it wants at the current price level.