All companies, large and small have a culture. A company's culture is usually implicitly expressed as opposed to explicitly. In some cases the culture is a reflection of a company's mission statement. A company's culture is a product of the personality and goals of the founders, the product or service being produced, key employees and the customers consuming the output of the company.
The culture is usually designed to keep everyone on the same page, enhance communication, convey an image, act as a bonding agent, establish an identity stamp for employees and to help recruit like minded employees.
Classic and well known company cultures are Apple, Microsoft, Google and Oracle. When you walk into these companies employees appear to conform to a dress code, communicate in a similar way, and have a similar work ethic and style.
Startup's usually are not much different from the big guys with the exception that the companies are smaller. The goal of the culture is the same.
The key difference is the size and number of employees a Startup has relative to a large established company. This difference has a big impact on the success of the Startup. Size matters in a Startup because a Startup needs a steady flow of new ideas to be successful. Very seldom is the first idea, product or implementation spot on. Startups need to iterate at a furious pace and need to encourage even the "craziest" ideas to be expressed by the "craziest" looking people even if the initial reaction is no way. Lot's of these ideas must be expressed in rapid fire succession.
Strict adherence to a monolithic culture can work against this requirement. A Startup actually needs to attract people that do not necessarily think alike and act the same to achieve the idea flow necessary to succeed. Larger companies can get away with a strict cultural bias because of their size and established revenue model. Also, The shear number of employees should result in some odd useful ideas being generated by the rank and file. (Yahoo might be the exception). Startups need dissenters!!
The challenge of course is to establish a company culture of civil disobedience that can effectively vet the new ideas and put them into action. A Startup that can do that is almost guaranteed to be successful.