Due Diligence typically involves researching a person or business entity in order to assess the viability of doing business with that person or entity. This research involves assessing their financial position including reputation, business practices, corporate and shareholder structure, sustainability and environmental impact, real property holding, intellectual property holdings and anything that may impact interactions with the company.
It is generally easier to find information on publicly traded companies, as the SEC requires that they submit information regularly about their business. A great deal of the information that can be found about companies comes from the SEC and an analysis of the company's market performance.