Amazon began as one of the first major online retailers, originally selling only books. It has since diversified into many other product lines and services to become the world's largest online retailer. Amazon offers both retail services like books, electronics, apparel, and non-retail services such as Amazon Web Services. As an e-commerce giant, Amazon faces high barriers to entry from economies of scale, differentiation requirements, and capital needs but medium barriers to exit due to specialized assets and contractual obligations. It competes with brick-and-mortar stores and catalogs but sees rental services as complementary. Amazon's strategic challenges include finding new growth areas and maintaining long-term profitability.