Hardly a secret - it's the first line of business listed in the first sentence in every year's annual report (see http://www.berkshirehathaway.com/2007ar/2007ar.pdf for example):

"Berkshire Hathaway Inc. is a holding company owning subsidiaries that engage in a number of diverse business activities including property and casualty insurance and reinsurance, utilities and energy, finance, manufacturing, services and retailing. "