When most people hear the term "financial sector," they think of banks. Although this is certainly the largest part of the financial sector, several other types of companies are included in the sector, as well.Best financial stocks for beginnersThese are some mature, easy-to-understand financial sector businesses that are smart choices for beginner investors:Berkshire Hathaway (BRK.A -0.75%) (BRK.B -0.51%) is not always thought of as a financial sector stock. However, it is an insurance compa...