There is very little difference between Berkshire Hathaway’s BRK.A and BRK.B shares.
BRK.B shares were created because Warren Buffett noticed that the BRK.A shares were growing considerable high is cost per share and that these shares were less obtainable by the average retail investor and could only be purchased by either very wealth individuals, unit trusts or mutual funds.
Thus, Warren decided to create BRK.B shares to provide the average investor with the opportunity to purchase shares in Berkshire Hathaway without the huge cost associated with BRK.A.
There are other very small differences, but the above is the only one that really counts.