Considerable information is available on publicly traded companies--those that trade their shares on public stock exchanges. However, most companies in the world are privately held companies.
Private companies are more challenging to research, since they are not required to publish detailed financial statements or annual reports. When researching private companies, you will not have access to annual reports, financials, analyst reports, or SWOTs. You will have to use article databases, directories, the company's website, industry associations, and patent information sources. In some cases, even these sources may not reveal much information.
Companies can also own other companies, called subsidiaries. If the parent company is public, the parent company generally files the financials, and incorporates financial information from the subsidiary within its own results. If the parent company is private, no financials are filed for either the parent or the subsidiaries.