Final answer:
Monopolistic Competition is the market structure where many firms sell differentiated products and compete on the basis of product differences, advertising, and sometimes price.
Step-by-step explanation:
The market structure characterized by many companies and some product differentiation is Monopolistic Competition. In such a market, firms compete with each other by offering slightly different products or services, engaging in advertising to create distinct brand images, and occasionally adjusting prices. Differentiated products are those that, while essentially serving the same purpose, have variations that distinguish them from competitors' offerings. These differences could include quality, features, styling, reputation, or customer service. Thus, in Monopolistic Competition, firms do not compete solely on price but also through other factors such as product differentiation and advertising.