Buffett has long said that Berkshire's "core" business is insurance

Underwriting profit was down 43 percent at Berkshire's insurance units through the first nine months of 2015 because of higher claims costs. While Buffett has long said results from those businesses can be volatile, he'll likely remind investors why insurance has been good for Berkshire over the five decades he's run the company.

Everything else

Increasingly, Berkshire's results are tied to a growing stable of businesses outside the insurance industry. Subsidiaries include well-known consumer brands such as See's Candies and T-shirt maker Fruit of the Loom, as well as more-obscure operations like chemical company Lubrizol and Israeli toolmaker Iscar. Berkshire also owns electric utilities and BNSF, one of the largest U.S. railroads.

Buffett always recaps how these operations fared and often gives some predictions about how they might perform in the year ahead. BNSF, for instance, spent heavily last year to improve service but is now facing a slump in commodity prices that's hurting demand. A stronger dollar could also affect profit at Berkshire's subsidiaries that operate outside the U.S. Buffett may also weigh in on what the plunge in oil prices means for his businesses.

Acquisitions

"Berkshire is now a sprawling conglomerate, constantly trying to sprawl further," Buffett wrote in last year's letter. He made good on those words in 2015. In July, he joined with  Jorge Paulo Lemann's 3G Capital to finance H.J. Heinz's tie-up with Kraft Foods Group Inc. A month later, Berkshire agreed to buy Precision Castparts Corp. for $37.2 billion.

Buffett is likely to give investors more details on why he bought Precision Castparts, which makes industrial components for the aviation and energy industries. He said in August that the deal would keep him from making another major purchase for about a year. Still, he could offer an update on his hunt for more acquisitions and whether he'll team up with 3G again.

Food for thought

Some of the most-discussed passages in Buffett's letters have little to do with Berkshire. That's because he often devotes a section to a business or investing topic that he finds interesting. Past reports have included thoughts on derivatives, stock options and public pension plans.

With China's economy slowing, a U.S. presidential election in full swing, oil prices at their lowest levels in years, and central banks in Japan and Europe implementing negative interest rate policies, there's no shortage of topics he could take on this year.

A blunder

Buffett is famous for his candor, and his letters often include some sort of mea culpa. In 2015, he pointed out how he waited too long to sell shares in Tesco Plc. The "thumb-sucking" cost Berkshire, which posted a $444 million loss on the investment, he wrote. Expect the billionaire to call attention to some new mistake in his letter this year.

See also:

Spend your day with Berkshire Hathaway (whether you want to or not)