A registered company/corporation is an organisation which is formed and registered with the appropriate statutory authority of the country as a 'company/corporation' in accordance with the corporate and securities law of that country.
Obvious to mention - all those companies which don't fall under the above category are called 'Unregistered Company'.
Examples of Registered Companies:
Microsoft Corporation - an American tech company. Develops and sells computer software, consumer electronics, etc.
HSBC Holdings plc - a British multinational banking company, which provides various financial services.
Robert Bosch GmbH, or Bosch - a German multinational company. Deals in engineering and technology services.
Reliance Industries Limited - an Indian multinational company. Renders services in retail, telecom, textiles, petrochemicals, energy sector, etc.