i'm learning about many dimensions of a corporation. Some companies deals with consumer goods - food, industrial resources while others deals with services - airline, hotel, insurance, hospital, etc. And some companies operate to deal with purely other businesses!
A holding company own a part of another company's stocks. Famous examples would include Singapore's Temasek Holdings and Warren Buffett's Berkshire Hathaway Inc. This list i found here includes many banks as holding companies.
A conglomerate is a large company which consists of divisions of often seemingly unrelated businesses. Some famous examples would include East India Company and a media comglomerate would be the Time Warner. Here's another list of conglomerates.